You don`t say what system you have for heating /supplying your hot water nor how old it is. e.g. an "instant hot water" sysem where turning on a hot tap causes the gas burner to flare up and heat water in a small tank which is then drawn off via your pipes and then tap. The cold supply to that system would come from the rising mains and thus provide pressure to the taps. That system often has a Winter or Summer setting - if that is yours is it set to the current season?
Or does your boiler have a hot pipe circuit that takes hot water via a coil inside a hot water cylinder and back to the boiler. That flow through the coil heats up the contents of that HWC and turning on a hot tap then draws heated water from that cylinder. The cold supply to HWC that would be via a header tank - and the higher the tank is above the taps then the greater the weight of water pressing down (trying to push out of an opened tap or shower head) and therefore the pressure coming from those taps.
The pipework to a basin tap would normally be in 15 mm tubing and that to a bath tap in 22 mm tubing. As you refer to the trickle being from just the hot - rather than low hot AND cold pressures - it is possible that an old system could have a scale build-up. Some areas of the UK are better or worse than others - remember the Calgon adverts for washing machine scale build-up???? Fernox do a bag of crystals that you hang in a cold water tank - lasts about six months - and that fights the limescale. I believe the bumf said it helps to remove existing scale too but it`s available via plumbers merchants, John Lewis`s etc.
Another thought - if the supply pipework has a peak in it`s length you might have trapped air. I did some years ago and tried a oft-mentioned DIY tip of connecting some tubing from cold tap to hot tap then turning both on (the attachments for fitting to taps and washing one`s hair came in handy. Fold the shower tube bit back on itself and wrap tape or an elastic band round to "seal" it), The cold water pressure cleared the bubble, I made a slight re-alignment of the pipework and all has worked perfectly thereafter.
The DIY plumbing tips (text and drawings / pictures) on this site will give you a good idea of different systems and therefore which your`s is. That may assist your identifying what is more likely to be wrong...Good luck, EyeTry
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