I'm in the process of leaving BT but thought I'd post this up in case it helps anyone else.
We moved into our current house in September last year. There hadn't been a BT line in years (NTL area) so a new line was installed from the pole and all new wiring to the house. We are on Infinity option 1.
I've also had a full house rewire, inc. a full CAT 6 network install throughout the house, back to a Netgear 24 port switch.
What was happening, was everytime I attempted to play FIFA online on the XBOX One, I couldn't find any opponents. The general connection to XBOX Live seemed fine, I could download updates and get online on Call of Duty games, just could not play an opponent on FIFA.
Tried all the usual port forwarding stuff, tried EA's support and BT's support, no one could establish an issue.
The HomeHub was also swapped out a couple of months ago, as I was having real issues with WiFi latency and the laptop.
Eventually with one last trawl through the HomeHub settings on the new one, I found that the YouView Zapper box was automatically creating some port mapping rules, presumably for the multi-cast for the BT Sports channels.
I removed this auto mapping, reset the router and power cycled the XBOX. Low and behold it was now working fine.
tl;dr - if you're getting 'No opponents found' on FIFA on an XBOX One, and you've got BT broadband and a YouView box, check your port forward settings and/or unplug the YouView box as a test.