Anything over a $20 bill has been hard to use in many/most stores for a very long time in the US (which seemingly at one point decided to freeze currency denominations in amber--see also penny--which I assume is permanent at this point as less and less cash gets used). But $100 bills at least were in pretty common use overseas. I still tend to carry some as backup cash.

They shouldn't have any problems in London, especially at places tourists typically frequent.

I once worked at such a place, and the till would be stuffed with £50 notes by the end of lunchtime.

I realise it's different north of Watford.