Rates are lower in the
UK generally speaking (because it's more on a per-use basis rather than fixed expensive plans), and service is better because there is more competition. Only Rogers has a decent 3G/HSDPA network in Canada whereas there are three or four in the
UK that do.
You might be better off buying a phone in Canada though simply because
taxes are lower, as long as it has no SIM restrictions on it, you can just buy a SIM in the
UK and put it in.
I've done it both ways, Canadian SIM in a
UK phone and vice versa, the problem is if the phone is locked by the carrier obviously. But it can be done if you get the right phone.
You can certainly text backwards and forwards, I do it all the time. You get charged more obviously for international SMS. Make sure you get the number right though, e.g. +19051234567 or to text to the
UK it would be along the lines of +442081234567
It's relatively simple to try it before you go if you're worried, just find someone using the same carrier you want to use in the
UK and get them to text you.
Steve.