T-Mobile coverage, you're asking?
Although I'm not too happy about T-Mobile's being possibly taken down from the UK market, I think Vodafone taking it over would be the best option of all. But anyhow, time will tell.
In regards to T-Mobile's coverage, I've seen my phone (E71 on T-Mobile's contract) without a signal in the Nokia Flagship Store (!) at Regent Street and couple of other indoor locations around Oxford Circus. It's not a brainer that 1800 spectrum is somewhat fussy in terms of providing solid indoor coverage, unless network is really dense, but that is not always the case with T-Mobile. While I generally don't like O2 (just don't, but still better than Orange!), I like their strategy of building quite dense network of microcells in town centres, and Voda does that too, but T-Mobile - well, rather rarely, at least from my observation.
I am also commuting regularly on Piccadilly Line between Heathrow and Central London, heavily using (or at least trying to) web'n'walk on my phone. There are few dead spots, that I wrote a piece about and sent out to CS a while ago (vide: http://adl.pl/tmobile_coverage.phtml), but as you would expect - no thank yous directly from RF departments were received, just a plain, automated CRM's reply from CS. Btw, Voda's coverage on that track is *way* better than T-Mobile's (this is where 900 spectrum kicks in, really).
Oh, and StreetCheck. Who actually believes in such things? These are usually useless, marketing tools to show how great we are. Go figure if you get a decent coverage within Nokia Flagship Store in the middle of London, using StreetCheck facility...