Good to hear you found where the packet loss is coming from but it's a shame if they won't do anything to improve it.
I don't have much experience with VPNs in a proxy type configuration, I've only used them to become part of an organizations network to use services on their intranet. There may be some setting that your VPN provider offers that can help though.
The short details are that the game and ACE (v1.0+) use UDP to communicate. The defaults are: game port 7777, query port 7778, and default ACE port of (game port + 2) so port 7779 on the london server. My guess is that either the VPN, your local NAT router, or another firewall are blocking some udp packets that the game server sends to you and ACE is detecting this as a connection issue.
NAT and VPN interoperability is a pretty deep topic but you might be able to change the VPN protocol you are using or find some settings to traverse the firewall. I would need more details to try and give more specific help such as which router and/or modem you have, which software firewall you might be running, and which VPN service provider you are using. Some packet capturing might also help to figure out where the problem is. Maybe this will be enough information already to point you in the right direction for finding a solution, I hope.
If you are able to solve the issue, I'd be really interested in hearing how you fixed it 🙂