15mm copper to 22mm for thermo shower valve

Spent the weekend fitting a new thermostatic shower valve to replace a mixer valve, and have managed to kill the flow. It's a low pressure system fed through the loft (cold water storage tank) and airing cupboard (hot water cylinder).

The existing pipework in the loft is 15mm, but I have connected the shower valve to these using 22mm (the valve inlets are 3/4in) in the hope that when i can afford to switch to a high pressure combi boiler system the pipework will be ready to go. The result is pathetic pressure.

Is this due to the 22mm pipework or something else? Any ideas?
Many thanks

What is the minimum working pressure of your thermostatic mixer?

How much pressure is available from your gravity system?
(working pressure in Bar is usually considered as 1/10th the vertical distance from your shower head to the base of your cold storage cistern measured in Metres).

The thermo mixer says minimum 0.1 bar, and the distance from shower head to bottom of cold water tank is approx 1.5m, with no room to raise it.

Pressure was acceptable through the previous mixer
Thanks for your response

a combi would have worked fine on 15mm