Hey, welcome to the forum!
Someone may correct me here but I've been doing quite a bit of research myself recently regarding broadband providers, as far as I am aware most providers are using the Openreach infrastructure, whereas VM has their own lines etc which is likely why you've noticed quite a big difference. There are 3 'gaming' broadband providers in the UK that I am aware of that provide optimized routing to game servers apparently, perhaps that is something to look into? I think you may even be able to get a second line put in so if you're getting high speeds with TalkTalk and a lot of account credit then maybe the cost of an extra gaming broadband line may be appealing. Overall though I do concur with your assessment, if nothing else has changed in your setup except location and ISP then that will be the cause.