I've seen them several times in the last month or so, mixed in with the morning traffic into LHR. It does look strange, almost as if a Ryanair or EasyJet were spotted on the US East Coast. Also looks a bit like that Blue BA A320NEO from a distance.
I didn't realise that anyone in Europe was using A321's across the pond, except Aer Lingus/Air Canada, but I always see them differently due to the shorter distances.