OK, well if the kitchen tap has good pressure then I would have thought that something between the boiler and the hot bath tap is the issue causing the restriction of flow. The way I see it, looks like either you or a heating engineer/plumber need to find the restriction. It could well be that you’ll need to remove panels, pull up floor boards etc to pin down what’s going on, but if it’s you who do this, find it, it’ll will probably be a load cheaper. There are most likely a few people on here that would have a crack at solving this, but would work best if they were actually at the property, as it seems to be just one of those cases. Would under the bath be a good place to start? I wonder what the heaty plumb people on here think?