It is not perfect, but most airlines will try not to swap plane models as it breaks all the seating assignments.

I think the older Boeing planes through 737-900 are as safe as ever. 737-MAX+ and 787 seem to be where the design process went awry. Even then, there seem to be known workarounds to the bugs.

For what it's worth, I still fly on them all, I just remember to tell my family I love them before takeoff when the safety card has MAX on the label.

It's unlikely that BA will put a Boeing on a London-Rome flight as they don't operate short-haul Boeing planes. Yes they could put a long haul on there, yes they could wet-lease another one, but those are very unlikely situations.

If you choose to fly Ryanair on the route instead, you are almost certain to get a Boeing 737.

If you care enough then you have to vote with your wallet, it's the only say you have.

If you go to the airline website (or even other booking sites) and click on details you can see the flight, operator, and the aircraft. In fact, some websites (I think kayak) can even filter by including or excluding aircraft specific models. In worst cases, you might need to know the aircraft from the seat map. From my experience they rarely change what’s the aircraft, if they had it written, most likely it is the aircraft that you will fly with.

Obviously it's not possible to guarantee but it's definitely possible to reduce your chances of being on a Boeing aircraft.

You can fly an all airbus carrier to avoid this scenario. JetBlue for instance in the US.