There isn't going to be a simple answer to this. We all know that if you are a light user then you should be on PAYG, and a heavy user should be on contract. In between the two, there is little to choose between the various networks.
You can roam on PAYG, but not as widely as on contract. The one exception is Virgin (who use T-Mobile) who offer a direct debit option on their PAYG package and that gives you close to a "contract" coverage.
The way to avoid roaming charges is to purchase a local SIM card. Then you can also receive calls from the UK without being charged for the incoming call (although this type of charging is supposedly going to come to and end - to much disgust of the networks who make a huge amount of money out of it).
For best coverage on PAYG, UK and abroad, Vodafone is probably the one. Virgin (on DD) might be better but they block a load of ports on GPRS which may or may not bother you.
Voda also offers fax (in and out); Virgin offers fax out, and few if any other PAYG networks offer fax at all.