Bit of a depressing read this thread to be honest, I'd even go as far as saying disappointing of the forum and some of its members and contributors.
The OP
@Angmering1974 has simply asked an initial question and appears to have been bombarded from all angles, indeed unprofessionally by one poster who appears to be a TPE employee.
Some of the posts in this thread appears to be (frankly irrelevant) personal opinions about the use of cash and the OP being behind the times. What's crucial is the rules as they are written, not the rules as some posters seem to feel they should be written (change rules for National Rail Conditions of Travel if you prefer).
The OP would be doing nothing wrong presenting a £20 or even a £50 note on every occasion they face the situation they describe, and some here would do well to remember that is perfectly within the rules and to stop chuntering, huffing and puffing nonsense.