I think history is the biggest teller as to why airlines haven't jumped on the bandwagon. The only domestic routes that will probably make money in the UK flying jets the size of the A220/320 are your core trunk routes, Scotland - London, Ireland - London, Bristol - Scotland.
Maybe these are the only routes that will make money FLYING ANYTHING.
is Hop in France heavily subsided?
If so maybe this is the only way the UK can have a regional network.