On the MSE (MoneySavingExpert) forum, there is a thread where someone wanted this, and back in 2008 T-Mobile did a 'family share' account but it was 55 quid a month. Someone else (again with T-Mobile) asked for a second phone to be added, and that had a separate number, but was all billed on in one account.
Sounds like you first need to decide what sort of money you are happy to pay, then check what phones you would get (or consider going into a contract with a SIM-only deal, so you can use a couple of existing phones, or buy unlocked at Asda Living, or [at somewhat higher cost] unlocked phones from some other phone store).
Then ring around the networks to see if their sales teams can offer you a deal. I think it will depend on what you want to do - if you plan to (say) have 2 phones, where one is for you (lots of calls, limited texts) and one for a DD (lots of texts, fewer minutes of calls) they migh feel they are going to lose out as many of these deals mean someone uses more of one than the other but pays for the option to use more.
Perhaps T-Mobile Flex is an exception, but of course now T-Mob and Orange have merged some deals / contracts might disappear.
Most phones are locked but Carphone Warehouse and their online stores like e2save.com etc may do so, but the cost can be quite a lot higher than Asda, though it depends on whether you need basic text + voice + limited extras (up to 150) or some sort of smartphone which could go up to 400 quid).