Can get 0% balance transfer at 0% fee previously but now there is less of them
But even if you have a 1% fee and it's an 18month period, you can get stick it in an easy access saver e.g. Marcus at 4.75% or ISA at similar.
The fact that you're paying in some small amount into it as minimum payments is minor, particularly if it's 0% purchases as you just top it back up.
I don't do it for the circa £2000 it makes a year but more as it was good for my cash flow position which was negative 40k for a couple years, and now it's easy money.
Yeah I haven't ever used more than 0pc fee.
But it's super rare at the moment