Due to most residential UK isps using American configured WiFi equipment...
... which use channel 11. Which partially overlaps channel 13 (
check it yourself). Adjacent channel interference can be even worse than co-channel traffic (from other wifi devices), so you might want to test if your current setup is actually better than if using channel 11.
And yes, it's a royal pain that Europeans use American channel layout and thus missing opportunity to have 4 (almost) non-overlapping channels instead of 3.