I would ring BA proactively and ask for your preferred rebooking option now. The connection is now under the MCT as you say, and it may be a while before BA’s IT picks it up. A 1 hour connection off a DUB flight would be very very tight as although you won’t need to go through immigration at Heathrow you will need to go out through customs and then go through security. If the DUB arrival comes in on a remote stand (which is fairly often the case as there are only two gates at T5 that can handle a DUB arrival) then it can be quite a slow process as you need to be bussed to a special stairwell at gate A23.