I recently made the mistake of meeting family members who flew in from the Virgin Islands (where they live) via Barbados. I believed they was coming into terminal 3 but it was terminal 5. Not wanting to be stung for paying for two car parking fee’s, I got them to meet me at terminal 3. My uncle used his credit card, his wife’s card didn’t work. TFL staff suggested she got a blue & white free transfer ticket. His card wasn’t charged, as far as I’m aware.
If they have a Contactless credit card *(uk or non uk) then they can use that. Also, if they use the Elizabeth Line then there’s a free transfer ticket that can be acquired from TFL ticket machines. However, it’s a longer walk from the Liz via the moving walkway to the bus/coach.
I don’t know about the Piccadilly Line but I’m guessing the same applies with the free transfer ticket. Most definitely a Contactless credit card* should work too. This is closer to the bus/coach terminal.