Terminal server performance over high latency links

Yep Citrix is the way you probably want to head with this. RDP's vanilla implementations are great for low-cost solutions under most high-bandwidth, low-latency environments for general desktop/windows app distribution. But the whole thing kinda falls apart under specialist workloads and high latencies.

I answered a recent, relevant question where I think there's some crossover, here: Improving Performance of RDP Over LAN

You'll definitely want to test thoroughly before splurging any cash, though, as you still may hit limitations with such a high ping.

Outside of the Citrix/ThinApp solution, you may need to consider decentralising some of your applications and moving them back towards the branch offices. Even if you can't move them right out to the branch, having a rack or some kind of presence in a datacenter in the UK or europe, and hosting your solution out of it, may be the best option.