There is no such thing as “long haul crews” apart from at Gatwick (Euroflyer vs BA)

Heathrow based crew do a mixture of short and long

they don’t get to pick who they work with so the idea that bad crew members are put together on purpose is silly. The likelihood of working with a crew member on one flight l/h there and back and then never ever seeing that crew member again is very high - I’d say 98% arbitrarily

it all stems from the in flight manager in my experience. I can tell as soon as I walk on the plane what “type” of flight it will be.

i know your question relates to l/h but on s/h if the in flight manager is greeting pax at the door upon arrival looking smart and relaxed, it will be a good flight ime. If the manager is on their knees behind the curtain upon boarding preparing meal trays it is usually a bad sign and a indication how the flight will be

there will be standards that have to be followed but - and I’m not a “BA apologist” - before anyone pulls that card whenever you mix humans and procedures and standards there is bound to be variation.
it’s the same in every industry. One day at the Tesco checkout you may receive great service the next you walk into a store the other side of the country and receive poor service. Is it right, no? But it’s what happens.

if you feel the service was not to your liking then it’s best to raise at the time with the inflight manager. Any shortcomings when raised this way are usually resolved. No point sitting in your seat getting more and more annoyed and wound up but remaining silent.

it is also true, finally, that a lot of crew are not interested in pax and just want to go abroad all the time and the same for lots of crew who are very bitter towards their employer due to past events so will do the bare minimum.