How can I fix low water pressure from loft storage tank?
Given that the tank position etc. is the same the most likely explanation is that you have a blockage or narrowing somewhere in the pipes - probably from some sediment you dislodged while removing the tank.

Is it just one tap, all the upstairs taps or are the kitchen taps also affected?

If it's the latter then the obstruction will be nearer the tank before the pipes split. If it's the former then the obstruction will be after the split or (more likely) at the tap itself.

If there's nothing blocking the affected tap(s) then you will need to flush the system somehow, but it will depend on what's causing the obstruction. You might be able to clear it by emptying the tank again, leaving the taps open and then using a hosepipe to force water through the pipes - but if it's a piece of grit this won't really help.