Three
Newer company but growing rapidly
Carphone Warehouse
Great company offering impartial advise and stocking phones from all of the major companies could save a lot of time
O2
One of the main players in the cellphone business
EE
One of the main players in the cellphone business
Virgin Mobile
Another piece of the Branson empire and as good as all the rest
Vodaphone
Tend to be a little more expensive than the competition but has the best network