In the worst case, they could just stop accepting prepaid cards other
than Oyster. The prefix of the card number identifies the type of
card.

That'd be a bad move for tourists as many of the cards which are issued
in foreign (to the tourist) currency are prepay Mastercards. Although
they are not ideal for a party travelling together as you require one
each.

The Travelex card (which offers the usual Travelex stupendously bad
exchange rates) is not contactless. Are there tourist prepaid cards
that are?

Also keep in mind that we tourists can and do use our regular cards.
When I was in the UK last year, I used my US issued AmEx card on the
tube and it worked fine, with one charge per day posted on the days I
used it. Dunno how many tourists know about it, but it'd be worth
some posters at the airport tube stations.