To clear up a few things on this thread:
CAA require that the passport and the BP be reconciled at the gate for all international departures of UK registered aircraft. Hence why all British carriers do this everywhere. However, there is no similar requirement for domestic departures, so that is left to the discretion of the carriers.

The BP check at security is not required to check passports for domestic flights, but simply checks you have a valid BP. Since it is not mandatory in the UK to own an idenity document, there is no way a security identity check could be enforced. As someone who flew domestically out of EDI every fortnight for a year, I never once presented a passport or any other form of ID at the security BP check, nor was I required to.

Some airlines require you to provide photographic ID at the gate on domestic flights. This is invariably for revenue protection, rather than security. For example, before EasyJet brought in this measure, they discovered they had created a market for their tickets. 'Travel agents' were buying up all the cheaper tickets and then selling them on at a mark-up to people who then travelled in the name on the ticket. To prevent this, the LCC's introduced photographic ID checks, to make sure the person who had bought the ticket was the person travelling.

BA and BD do not require any ID at the gate, but on occasion may require ID at check-in (but not always - it's perfectly possible to go through with baggage only on BA and not have to provide any form of ID). BA accept a BAEC card as ID, which is neither photographic nor contains any information not already on the system, so it's obviously a very worthwhile identity check