To be honest, new kit will be a pretty similar price wherever you go. Caveclimb.com is in Cheddar, so probably your best bet. Make sure to call ahead as Andy opens to order. Otherwise Starless River is a mobile caving shop that visits the Mendips fairly frequently (possibly this weekend?).
While oversuit seem pretty expensive, they last a while, so pretty cheap over time
Alternatively, if you want to save a few quid and aren't in a hurry, keep your eye out for second hand kit on ebay, it comes up from time to time...
