I'm just back from two weeks in London and, since this issue comes up pretty often on the forum, am sharing my experience.
It's true, you don't need money anymore in London. Well actually you need a ton of money since it's pretty expensive, but you don't need cash. Everything is tap to pay. Restaurants, the Tube, the loo in Covent Garden (careful there isn't much privacy), donating in a church - all done with a card. So make sure your card has that tech (by now it should, but until just a few weeks ago I had one that couldn't tap). The only time I used my cash was to tip the chambermaid at the end of our stay, so I'll be going to the currency exchange next week.
MC and Visa are obviously the most popular cards, Amex is accepted but not everywhere (heard an argument in a restaurant about no Amex). Follow my usual advice to bring several cards of different types from different banks, because at some point for no reason one card or another will stop working. My wife's card started acting up in the Tube station so we switched to another. Be sure to use the same card to tap into and out of the Tube.
Restaurants will bring a small terminal to your table and you'll pay with a tap, however if your bill is high enough they may have to print out a receipt for you to sign (this happened in a taxi). Your bill will almost certainly already include a "discretionary" service charge of 10 or 12.5 percent, which is a convenient way of handling things; it's actually refreshing to not have to poke at a screen selecting a tip percentage while they watch as here in the States.