Some £100 notes are circulated by some Scottish and Northern Irish banks (which are allowed to print their own currency), but given how rare the £50 note is, I'

Some £100 notes are circulated by some Scottish and Northern Irish banks (which are allowed to print their own currency), but given how rare the £50 note is, I'd expect the £100s be very rare even locally, and almost non-existent anywhere in England. They almost certainly wouldn't be accepted anywhere except banks (many shops are suspicious of even low value non-standard notes), so I don't know if you can really call that "in circulation throughout the UK".