This from a friend who lives in the U.K.
I carry very little cash these days and use it even less.
During the original Covid outbreak we were encouraged to use contactless card payments and they can be used just about anywhere now. Even ice cream sellers in the middle of nowhere will accept them.
I would suggest that say £50 in cash would be more than sufficient which could be topped up from an ATM if need be. I use my current (checking) account card to get currency in the US without any problem.
We were in Spain on a cruise last year and they were more than happy to accept my Mastercard for a beer and a coffee so again €20 or €30 should be more than enough.
As far as tips, there is little tipping in Europe.