The United flights that are quoted in Europe all have 1 major thing in common, they are all big cruise destinations. Sadly as far as US leisure traffic is concerned the UK is purely London or to a lesser extent Edinburgh.
places like MAN, BHX etc have to rely on ex UK passengers ,which as far as USA is concerned is a long long way off pre pandemic levels. Added to which unless you live within 10 minutes of your local airport the attraction of saving a few quid and flying from say LGW is quite an incentive to your average UK holidaymaker at the moment.
I actually feel ,being a Brummie that MAN has always punched above its weight on US flights in the past and think it will return ,however i think the whole aviation industry has changed at the moment and nothing is certain at all