The only difference between being a BT customer vs Sky for FTTC should be the backhaul, ISP network and peering.
You really need to be able to engage with someone from BT Network OPs to figure out the problem. This might be a bit tricky....
Backhaul - Telephone Exchange to ISP's network (BT Wholesale's 21CN network for BT Retail/Plusnet)
ISP's Network - Different network devices (gateways/switches/routers) your traffic passes through
Peering - Essentialy what connects you to the internet and carries your traffic. ISPs will pay different peering providers so you can access the internet.
You really need someone from BT to step forward so you can give them information they can process/analyse.
With your FIFA lag, is it all the time or specific time? Do you notice any other slowdowns/problems apart from the lag?
Depends what you mean by 'connection' though. It's essentially Openreach equipment rented or bought by Sky.
ADSL is a little bit different.
Not quite sure what you mean.
VULA is OFCOM lingo.
The Openreach GEA products (GEA-FTTC/GEA-FTTP/GEA-Multicast) are VULA products. It's a stop gap until you have full LLU for NGA, but it allows all ISPs access to the NGA product range using Openreach's network.