Are you planning to hit the "continent" in addition to the UK? Because the Eurail doesn't work in Britain, you have to have a BritRail pass. I don't recommend those though-- just take the coach. National Express
has frequent, inexpensive coach journeys all over England, Scotland and Wales. And I think they may go to Ireland too.
As far as continental Europe goes, I don't think a Eurail pass is necessary. But it does make life easier in some cases. There's a company called EuroLines that has bus service all over the place, but honestly, for the amount of time it would take to be on a bus from, say, Rome to Paris, it may be more worth your while to shell out the extra dough to take the train and get there hours and hours quicker.