It would be a logistical nightmare to put a different aircraft (i.e. non A321) on that route as a one off, as they'd need more cabin crew and different flight crew. This isn't feasible with a last minute tech issue. I think there are only 4 of these mid-haul A321s left, and they do DME/AMM/BEY and sometimes SVO/CAI/TLV, so the fleet is spread very thinly.
You should get the standard World Traveller service on AMM, so IFE and a hot meal - it's definitely worth contacting BA about it. Just be thankful you didn't book a J ticket!