Paul Corfield wrote
[...]
Do TfL advertise appropriate means to pay? (e.g. not £50?)

They do advertise the limits on legal tender re coinage in the
tickets
booklet but I have never seen a statement limiting the proffering of
bank notes for payment.

Wouldn't help since change can always run out, whatever float a driver
or ticket office starts with.

I remember that BAA or LT once had words with the Chinese embassy when
it became apparent that official parties were thoughtlessly being given
individual allowances consisting of £50 notes only with consequent
embarrassment at Heathrow

I don't think anyone asserts that there is or has ever been an act of
parliament requiring change to be given, though ISTR an assertion that
one ? railway comapny before WW1 had notices "booking clerks are
neither required to provide change nor authorised to refuse".

A workable rule might be that drivers could accept cheques if they had
no change for the next note up or otherwise give a receipt for
reimbursement by post . Foreigners and those who have no bank accounts
would still be stuck however.

--
Mike D