Indoor coverage vs Outdoor coverage
About a year ago I moved away from EE. The reason was simple - indoor coverage was extremely poor (and this was in spitting distance of EE's headquarters in Paddington). Forget speed, I just wanted a signal without having to lean up against a window. Now I'm with O2, I rarely suffer signal issues.
So, I'm not sure how Ofcom got their results. Perhaps they drove around measuring speed outdoors and then extrapolated (probably incorrectly) for indoor coverage, or perhaps they put an app on volunteers' phones to sample data speeds. But then most people would select a mobile carrier they could reliably use, and this would minimise readings that negatively effect results. Whatever the method, it certainly doesn't reflect my experience of EE.