UK Special Branch have said (in the 1980s) that they want an informant on every street. Duncan Campbell (who released information about eg ECHELON 30 years ago) has a great book about pervasive UK surveillance.
It does not surprise me that law enforcement would want to keep tabs on the more extreme elements of the environmental movement, any more than that they'd want to keep tabs on the more extreme elements of any other political or religious movement. (Unfortunately, it can be hard to filter the truly extreme from the loudmouthed-but-harmless, and there has probably been a lot of wasted effort over the years in infiltrating groups and surveilling individuals who weren't at all dangerous.)
Essentially you had lots of surveillance teams and analysts who had nothing more to do, because the Irish terrorist threat had pretty much evaporated over night.
They kept watching the Russians and some internal SOCA type targets (organised crime), but mostly there were lots of people twiddling their thumbs.
Alongside this, the animal rights campaigners had done things that were vaguely "terroristic" in nature. Since the target of the animal rights campaigners were often the elite who hunted foxes, the hunt group lobbied to make sure these groups got Special Branch's attention. Special Branch like paid overtime, hence they got someone new to focus on. The hunt campaigners won however and hunting with hounds got banned. Though it must be pointed out that a large number of the public in general favoured the ban by 2004: https://en.wikipedia.org/wiki/Hunting_Act_2004#Public_opinio...
Now nearly everyone is working on the Islamic desks. Some Russian stuff still and maybe 1% on the Irish to keep an eye on them, but the large Irish informant network will have been left to rot.
As a side note, the Security Services have traditionally had difficulties building informant networks inside the Islamic targets (inside the UK). The Irish responded well to payment in cash and drugs and "look the other way whilst we continue our Mafia-like activities", since for the most part, they were a well armed and well funded crime syndicate with a religious hard-core. Islamic fundamentalists haven't responded so well to offers of cash for information.
Also in terms of putting officers inside these groups, the UK has a very small minority of officers that could actually blend in undercover, although the trend is increasing. Total minority ethnic police officers made up 5.5% of all officers on 31 March 2015. There were 6,715 Minority Ethnic police officers in the 43 police forces on 31 March 2014. This represents 5.2 per cent of the police officer total. Only a handful of officers become Special Branch officers and they are hand picked (I believe unofficially). The chances of a having a Muslim minority Special Branch officer, prepared to spy of other Muslims is therefore rather slight.
https://www.gov.uk/government/publications/police-workforce-...
They were a major target in the 1980s when they were firebombing department stores.
Anti hunt campaigners got increased attention when the Hunt Retribution Squad started desecrated a grave with the intent to remove the corpse and send the head to Princess Anne, again in the 1980s.
There was some crossover between the groups. Increased restriction on rights to protest and increased protections of businesses have probably created more extremism. Now instead of campaigning against a laboratory using animals the protestors will campaign against the companies who supply the lab - companies selling stationary or office equipment or chairs, or supplying admin staff.
Here's a different group from the 1980s.
https://news.google.com/newspapers?nid=2507&dat=19841227&id=...
They tried to dig up a corpse in order to send the head to a member of the royal family. That may have been posturing, but it's obviously going to draw law enforcement attention.
Here's another group that did dig up a corpse in order to blackmail a family.
https://en.wikipedia.org/wiki/Save_the_Newchurch_Guinea_Pigs
There was a bunch of firebombing activity in the 1980s. The animal rights activists said the intent was to create a small fire that would be discovered by smoke detectors, triggering the sprinkler systems, causing water damage. This happened in department stores selling fur coats.
Animal Liberation Front pretty much started in UK, and some people claim it's caused many millions of pounds of economic damage. There are unacceptable levels of harassment and intimidation in some recent animal rights campaigns (and this is partly driven in changes to law which make protesting less legal).
So, police have these spiky things which they probably should be investigating. But it's hard to tell when someone has a "Rats Have Rights" mug whether they're a peaceful protestor using strictly legal means, or if they're going to become radicalised and start doing actions under the name of ALF. And police clearly didn't understand the mostly youthy animal rights movement in UK.
Anti hunt campaigns had uneasy alliances of anarchists, class war activists, and animal rights campaigners. Class War caused extra attention because their newsletter had "page 3 beauties" - photos of police officers who'd been hospitalised after violence, and they'd made many statements about being prepared to use violence against people. (Hunt saboteurs had strict rules about not using violence against people. There was some debate about whether it was okay to use violence to defend yourself if you were attacked by hunt followers.)
There were a bunch of legal, peaceful, campaigning groups that tackled things like vegetarianism and veganism; vivisection; primate rights; farm animal welfare. There were a bunch of groups that appeared to be peaceful but which were used as feeder groups to more extreme groups. (EG the National Front and British National Party set up an animal rights group, campaigning on ritual slaughter, to drive people to their fascistic groups.) And then there were the obvious activist groups who would raid laboratories. Some of those groups tried to use legal loopholes - they would enter a lab, not cause any damage or take any animals but take all the paperwork they could find, then scan and copy as much of the paperwork and then return the papers. Theft requires the intent to permanently deprive the owner. That tactic didn't work, they were caught and prosecuted, and so activists went back to liberating animals and causing as much damage as they could.)
If you're looking for an at the time account you could try to find back issues of "ArkAngel", which had debate and information about activist action.
Later they got backlash for stealing dogs, and not the rats and other un-cute animals, so they invaded again, and released all the other animals.
Then people started to notice that the stolen dogs were being released on the streets too, with the activists realizing they could not care for the dogs.
The laboratory if I remember correctly was working with antibiotics and some other medicine, meaning that people spread in nature diseased animals, or animals that needed medicine to survive anyway.
Those animals are very destructive.
See ref role of "Anna" here https://en.wikipedia.org/wiki/Eric_McDavid