TBH I wouldn't sweat it too much. I came back from 3 days in Edinburgh with a 1.5 loss and I was there to enjoy and not say no - cant have.
The good choices were Marks and Spencer salad/pasta lunch meals - I did pick the ones that were low fat, low sugar - usually beans or couscous based with salmon or feta. We arent people who have to eat certain meals at certain time, we eat when we are hungry so we often skipped breakfast or got an egg McMuffin if we knew next chance to eat was going to be really late.
I did have wine, I did have an Indian meal, I did have fish and chips , we also walked around 10 miles a day :)
One thing Id say, is download the Too Good To Go app, you can pick up all sorts of on the go type food for very little money . Even the hotels put up whats left from their Buffet Breakfasts at 10:30
Theres loads of parks in London if the weather is dry, to sit an eat. When Im taking friends over one thing we do is the river trip - planned for lunchtime - so we sit and eat whilst getting a commentary on the sights up the Thames
And whilst Wetherspoons isnt fine dining, its cheap and easy and they do have healthy options, as well as catering for other dietary needs. I had the fish and chips in there, rather then a chippy. Small portion, oven chips, oven fish, and a side salad so whilst off plan, I made it the healthier option
Dont forget, one meal isnt going to break the plan, just get back on as soon as you can. Enjoy your time away