Chatting as in text chat? Well, if both of you don't mind signing up for a data plan at both ends, I'd say easiest way is to go blackberry. Your GF can probably get a BB for free in the UK, with a 1-2 year BIS data plan and you can always sign up for a 1-2 year contract with a telco here too.

My recommendation - BB Torch 9800 with Digi Smart Plan (+BIS). Relatively cheap (phone) and very simple/easy billing structure.