That’s correct, they are legal here in the UK, although organisations such as the RSPCA aren’t keen on them.
I also found a great Trusted Housesitters blog here, it’s more about dog harnesses vs collars, but it also discourages the use of choke collars and prong collars.
I think @PNW has the right idea of passing on any sit where use of a prong collar is used, if you’re against using them.
There was an interesting discussion a few years ago around various types of collars, with a suggestion being that information about any specific type of collar being used should be shown in a listing as standard.