In reply to snoop6060:
I tried EE briefly a few months back and had to cancel the contract because they couldn't provide any signal at my central Edinburgh office location! Probably just an anomaly, though.
It's worthwhile being aware of the difference between getting a signal, and getting 4G. Last time I looked Ofcom showed Vodafone as having the widest signal coverage across the UK, but I know from experience in the West Highlands that having a strong signal doesn't necessarily mean getting decent mobile data (AIUI Vodafone is fairly notorious for having poor data backhaul outside of urban areas.)
So it depends what you want it for. If you need a phone that you can use to call for help in remote areas then look for vanilla signal coverage to give you the best chance of getting voice or SMS communication. If you want it for mobile data in your rolling palace, sorry, office, then I think EE are currently top of the pile, barring the odd perplexing notspot.
Other alternatives, as suggested above, include a roaming SIM (but I imagine the cost would likely be higher than tying yourself to a single network) or a suite of PAYG data SIMs (although you might find swapping SIMs gets a bit tedious after a while, plus they aren't really designed to stand up well to regular manhandling).