I like to use my cards because I get points for the purchases so it is a win-win for me, but some people don't do that with their cards.
If you are asking about whether or not you will NEED £'s while you are there, here is MY personal experience:
Having just left London/England in the last 3 weeks, I will say, we took $250 cash that we had converted months before here into ££'s to use for tips in our room/at cash only spaces. We came back, after 17 days with over £150 so I recommend only getting it in small bills from your bank (if they have a good exchange rate, which mine does) and plan to use that for the tips you know you will use and maybe £20-30 over that for any incidentals that don't accept cards. I would also get it in small bills/coins if you can. Our bank gave us an asst from £20-5's so we had to "break" them for smaller coins and bills at various spots.