This is a direct copy and paste from my post on an AB thread about tipping, back in 2015:
"Around 20 years ago, I was on my own on Christmas Day and decided to try out an Indian-East African fusion restaurant in south London that Time Out said would be open. I had one of the dearer starters, a higher-priced main course, a dessert and two glasses of lassi. The food was all freshly-prepared to order and was possibly the best meal I've ever had. The company was delightful too, as everybody was talking to complete strangers on other tables in the restaurant. On a day when my local pub would have charged me £35 for a bog-standard turkey lunch, my bill came to £10.80 but the restaurant owner then rounded that down to a tenner! I happily paid that tenner - and left a twenty quid tip on top!"
Despite being on my own, and not even having a booking for the restaurant, I had one of the best Christmas Day's possible, due to the fantastic restaurant staff. If was well worth the two hundred miles driving to get me there and back, so I didn't begrudge leaving a decent tip ;-)