Low pressure upstairs when downstairs taps are valved off?

We have no / water pressure upstairs. The kitchen taps downstairs are capped off as we are fitting a new kitchen. When this happened before, I opened the valve to the kitchen cold tap and the pressure increased immediately. That is not an option now.

We have a cold mains feed to the kitchen. Pipes then run up to tanks in the attic and back down to bathroom.

Not a mains problem - outdoor tap off the same feed is fine.

S