Actually polymer (not plastic) but these longer lasting bank notes are called 'plastic money' and the UK finally joined over 30 other countries using them a couple of years ago and the last note to get them was to be the £20 note in 2020.
The £50 note was to be left as is as it was not used as often, but now it seems it will be going plastic also now and the public are being consulted on who should appear on the reverse of the note.
This will make all current British bank notes new polymer notes in 2020, joining the £5 and £10 notes that are in circulation already.
The UK bank notes can always be returned to the Bank of England, even if out of circulation and decades old, (I have some myself that need to be done).
The highest denomination in the UK is the £50 note. The £100 note was removed some time ago as it was hardly used and to help stop its use in criminal activities.
Have you ever owned, held or even seen a £50 note in your lifetime?
In the USA, the $1,000 dollar bill was removed from circulation for similar purposes I believe. As only a thousand of them, a thick bundle, would be equal to $1,000,000 (1 million dollars). I believe the $100 is the largest denomination in the US currently and there is even a £2 bill, which is available at banks on request, but is not widely used. The $1 bill is more than incredibly popular, it is also very convenient. So much so, that every time the discussion of removing the note in favour of a coin replacement, receives unhappy responses from the wider public, compared to the convenience of the dollar bills that fit into payment methods from meters to vending machines, means the 1 dollar bill will remain in service for many years to come.
Fact you may have heard before:
It is also been said for some time now, that every single dollar bill in the USA, contains some trace amount of illegal substance on them, such is there widespread use and prevalence of drugs and drug dealing in the USA and changing of money in this use.
Australia invented the polymer (plastic) notes and were the first country to issue and use them, more than four decades ago.