Based on info fairly easily findable by googling there are at least two prep schools in the UK that will take boarders from age 6.
Obviously Henry would be far better off being brought up single-mother-of-two Helen, or (if Helen is otherwise engaged) by Pat & Tony as Johnny's little "brother"; but if that's not an option and Arsula & KnobDaddy are willing to pay, then Henry might be better off at boarding school than at Blossom Hill cottage learning to be an abusive mini-knob, while Knob and Arsula are still there. At least at boarding school he might get sympathetic and sensible houseparents that he could be allowed to bond with... and Pat & Tony could visit him independently.
[removes rose-tinted spectacles about bullying at boarding school... and hurriedly puts them on again...]