Trust me, you want it

The British Heritage pass covers pretty much everything you want to see outside of London.  (And all the London museums are free, anyway)...  If you see more than a few major sites (Stonehenge, Shakespeare's House, the Baths in Bath, etc), you'll be saving yourself a lot of money and a lot of hours waiting in a 'queue'!  You can buy it before you get here, or pick it up at a select few Tourist Info centres.