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A British VPN company would be trivial for the government to attack directly using legal powers. An overseas one would be much harder.
As for the tech argument: In theory it's a lot easier for a state actor (the UK) to monitor all traffic if all the traffic is kept within the UK. There could be timing attacks on a VPN to observe traffic going in and out, and if done during a quieter period they could correlate "that home/mobile IP uses the VPN to access these websites".
You want to make their job harder, surfacing your VPN elsewhere does so.
Top tips:
- Don't use a VPN run/owned by a UK based company.
- Don't surface in the UK during off-peak times if you're really paranoid, but you're probably OK if you've used a large provider (like PIA).
- Don't use a VPN run/owned by a UK based company.
Don't use a British VPN.