You are not expected to tip in the UK! I'm not sure why some people think otherwise.
I would only tip if service was above and beyond but it is not an expectation and I wouldn't "cave" if it was crap service.
Husband and I had a rubbish meal at a well known resturant chain for our anniversary. I complained at the time but nothing was done about it.
Left husband to pay the bill and he tipped them. Bloody hell, I want my fiver back!
Seriously considering LTB over this.