Don't forget the £10 delivery charge. There are 2 other differences though, theres no manufacture warranty which can cause problems if the lens is an import as well as a white box item. I dont know if they supply a VAT receipt now, I didnt get one for my 24-105mm last year. The warranty situation is better with an L, they have international Canon warranty.
Only £18 difference after delivery charge is included, next cheapest UK retail price is around £175.