VPN is 90s mate. Can't they figure out when u connect to VPN. i know ISPs does in countries like middle east where people use VPN to make Voip calls and they start filtering packets the moment u connect to VPN. So thats ruled out
I'm thinking more Western Europe and the US, where VPNs are common place and not frowned upon (people use them for work etc.).
Once your data is going across a VPN ISPs cannot check the nature of the data, just the amount of traffic.
VPNs are certainly not 90s, they are alive and well and used all the time (ask most of the people here into international media buys).
But I am not 100% sure about BTGuard (the proxy version not the VPN version) as I would imagine that they would see the proxy address and start to filter it out. But not sure.