Ive had enough of o2!!!

Get a couple of PAYG sims and test a couple of others before you commit.

definitely Three, Orange or T-Mobile... Usually Three but thing is T-Mobile use Three's masts as part of MBNL in addition to Orange's own masts, and because orange use t-mobile as well they are the same. id say go with either orange or t-mobile. im on orange and i never dont have 3G o2 is poor for 3G covergae

T-Mobile.. from experience. However they do cap speeds on some tariffs so Orange might be a better bet is that's an issue.

Despite 3/T-Mobile/Orange sharing some masts, it's not every single one. In my part of London 3 share with T-Mobile on two masts, then have two of their own and Orange still have their own GSM/3G up too.

I suspect the reason that 3 have more masts in London is because that they have far more data useage on thier network than the others, it's not really a coverage issue per say it's more to deal with peak times. As overloaded masts start to shed users by reducing coverage to off load to other masts.
So at peak to keep you need more masts to keep the same coverage if you have a congestion issue...

Can someone confirm this for me: Orange & T-Mobile have more 2100 frequency allocation combined than the other networks singularly? Allowing them to deal better with population saturated areas like central London when they combine the network later this year..

Vodafone is terrible in London and 3 probably the best but O2 in London is pretty good with the 900 frequency

Id say Orange is best because not only does their own 3G cover 93% of the UK they can now share with T-Mobile and they use most of Three's masts for 3G (An agreement called MBNL). I am with orange and I ALWAYS have 3G and speeds are great, I have seen 8mbps!!