I get this will probably be a 'cancel the cheque' thread (skimmed), but if you pay £20 & say 'so that's £2.50 change you owe me, but if it's easier I can just give you £15 next week, or I'll give you £20 each week for 3 weeks then a tenner the 4th week, whichever you prefer' then surely that's problem solved?
She's getting her £17.50 & advance payment on next week, so all good?
I think it's reasonable for her to ask for cash, but then the onus is on her either to make change, or, if she doesn't have it, accept that you've paid a bit towards next week in advance.
If that wasn't good enough I think I'd be looking at making other arrangements.
I pay my housekeeper daily & quite often say 'sorry I've no change so this is for tomorrow as well'. On the one occasion I couldn't give her cash - I was out & had tasked teenage ds with paying her, & the local atm was broken - she wasn't happy, which is fair enough, so I rang a neighbour to pop the cash round.
But if you pay her at least what she's owed & possibly some in advance, I don't see the problem.