I know this is a delicate subject, but please do not do this. We have our own tipping system over here, and too many American tourists applying their own system can only lead to changed expectations and erosion of our way of doing things.
The first thing to be aware of is that we have a minimum wage in this country, and that includes wait staff. You are not effectively paying their wages like you do back home. I know this will make you feel like you are being mean, or even stiffing your server, but please do it our way. And that includes no tip at all for lousy service - remember the server is still making a wage.
As a rough guide, around 10% if your happy with the service. If you’re in a Bella Pasta type place where a couple of plates of pasta and a couple of drinks will run to say £22, I would normally pay for that using a card, and then chuck a couple of pound coins on the wee plate that the mints come on. If in a pub where they let you put two meals and two drinks on a bill and it comes to £18.50, I would normally hand over a £20 and say keep the change. In a higher end restaurant, 10% and round up would be the norm, as is adding the tip to the card. So a £76.20 bill I would round up to £85 on the card for normal service, £90 for good service, or pay exactly £76.20 for crap service.
I’ll now duck and run, and await comments about stingy Scots.
Anyway, enjoy your trip, don’t worry about standing out, and just go with the flow when things turn out different. I remember my first trip to the States when I ordered a steak sandwich, which over here would be two slices of Sunblest with a bit of frying steak and salad on it (I don’t have a huge appetite). I just about died when an 8 oz sirloin atop four slices of toast (with flags in it), a mound of fries and salad in separate bowl turned up. But I knew to tip 15-20%!