I still think it is illegal for those other agencies to spy within the home country. Not to mention countries they don't have agreements with.
The home agency does not do anything illegal itself, but the other agency does illegal stuff for them.
I generally feel positive about the law but I'm curious how this feels from a position of paranoia. (Very tangential followup this is just the first time I've seen intelligence agencies and GDPR mentioned at the same time).
This is a mixed blessing.
Laws like RIPA (in the UK) sound draconian and scary, but they do mean that almost all law enforcement activity comes under a legal framework and that there are checks and balances on use of investigatory powers.
The UK has more CCTV than any other country in the world, but now CCTV is regulated by the information commissioner. We even have a surveillance commissioner to look at what the security services are doing.