The merchant can also disable you from selecting 'credit' while paying to force it to use the debit network.
Its also like this in Australia.
Very few banks still issue debit only cards (Switch/Maestro), they do if you ask for them, but they mostly only work at the ATM.
This never happens in the UK. It confuses the heck out of me in countries where it does (e.g. New Zealand).
1. CREDITO
2. DEBITO
where you choose if you want to pay with credit or debit.
Last year I went to the UK on vacation. While paying my bills I noticed most people wouldn't know what to do when prompted. They would look confused and just hand me the card machine and let me handle it.
Same thing in Singapore. Never need to select credit in NZ / Aus / SG. Unless you specifically want to use credit.
I don't think any UK banks have their own payment network. The cards you get to access the account directly are debit cards (usually Visa Debit or Maestro), some (i.e. children accounts) give you a 'cash card' which only works at that bank's ATMs.