I think my house has a water flow rate problem and I'm looking for a
solution.

The house has a combi boiler and everything is run off the mains water.

The water flow rate from a single cold tap upstairs is about 10 l/min
The water flow rate from a single hot tap upstairs is about 8 l/min

The difference is probably due to the resistance of the combi boiler.

The problem is that if you turn on a cold tap the hot water flow rate
reduces.
If you turn on a hot and cold tap the flow rate is about 9.5 l/min from the
cold and about 0.5 l/min from the hot.
The low flow rate of the hot causes the combi boiler to stop heating the
water due to inefficient flow rate.

Solution Ideas, will the work?

1) A pressure equalisation valve on the hot and cold water supplies?

2) replace the cold water supply pipe to the main pipe in the road as the
input in to the house is only 15mm
Not really an option.

3) run the cold of a tank in the loft that is pumped?

4) run the cold and combi of a tank in the loft that is pumped?
Before the combi was installed by the previous owner there was a cold water
tank in the loft.

5) Any others?