I get literally full 4g signal around most of both westfield shopping centres.
The signal goes a little in the stratford westfield cinema agreed, that's just because the cinema is made from metalic materials, you're basically sat in a metal and concrete box. The O2 signals ok here as the mast is closer by probably. That said O2 often goes to gprs in London for me.
A40 I have no issues.
The frequency O2 use means their 3G can penetrate concrete better as it can travel further but EE have kind of counteracted this by having much more masts than O2.
Overall they're both very similar in my experience. Due to mast layout some areas have black spots where others don't.
Edit: can honestly say I only don't get signal in central in one or two locations. Often a switch of airplane mode off and on makes the phone switch from a weak t mobile signal to a very strong orange one. Once EE integrate everything into tmobile standards this will go away.
Edited by ukhardy07 (Sun 02-Feb-14 15:37:54)