0: http://www.slideshare.net/Computaris/eu-roaming-regulations
The only reason customers would sign up to ARPs is to avoid expensive roaming charges. It's been apparent since before the ARP/LBO legislation came into effect that the EU was planning to eliminate roaming charges altogether within a few years. So, potential ARPs were faced with the prospect of operating for just a couple of years before their market effectively vanished.
It also didn't help that the mobile network operators dragged their heels on entering into agreements with the ARPs that did try to enter the market.
3LikeHome lets you roam in 18 different countries, the data used is taken from your allowance, and you can call home for normal price.
I am a dane living in London, and even though they say it isn't for longer stays abroad, I have been using my danish number in the UK for over a year without any issues.
I guess there is no reasons for alternative roaming provides when regular phone companies makes these changes themselves.
I'm emigrating in August, and my experience with Three.co.uk in London was terrible (congested network, impossible to load a map or email when I most needed it).
I had issues in Varde (close to Esbjerg) a few years ago, where the connection was bad, but that might have been fixed.
They have a map here: http://80.251.200.40/coverageportal_dk (sorry the shady link, that is what they provide)