I tried manage my booking with no success, just a message to say the return flight is cancelled, I can imagine the wait on the phones after easyJet!
Check online at ba.com for the current call Centre opening hours. Start calling about a minute or two before the published opening time, and by the time that you get through the announcements and menus, you’ll be in the call queue the moment that it opens or very shortly thereafter.
You should then be on to an agent quite quickly.
You can have a full refund, which will also include your seat selection fees. These will almost certainly arrive separately from the airfares themselves on your card statement.
Alternatively, you can ask for a rebooking, either on BA on a different date if it’s available or, ultimately, on another carrier. You may get some pushback on this, but it is a legal right.
Personally, and unless you got a very good fare at the time of booking, I’d take the refund. That keeps your powder dry and means that you don’t have to call again if the new flights are also disrupted.
It also means that if, by some miracle, you are able to travel, you may well find that there are some aggressive fare deals to try and stimulate demand which is, still, absolutely on the floor.