right have you hecked the flexi hoses coming off the tap arent twisted? as the slightest kink/twist can cause them to give you problems,

is your hot water fed from a combi boiler direct or a cylinder indirect? easy way to find out is do you have a big hot cylinder in your house or not?

is the bathroom upstairs or downstairs? as if upstairs and hot water is fed from a tank in the loft through your cylinder your not going to have very good pressure anyway, therefore you would be better off with some kind of pump setup or electric immersion heater for upstairs coming off a cold main,

does the tap have a british standard mark on it something like bs5544 atall? if not im suspecting the tap mixes inside instead of at the end of the tap and the cold pressure is pushing the hot pressure back downthe pipe if the hot is fed from a tank/cylinder indirect setup, and therefore a non return valve on the hot might help that out,

fitting a bigger isolating valve i would suspect will do nothing in solving a pressure problem when it was fine before hand, fitting a new one might if the old one was nackerd and staying half closed