If I understand you correctly – you are fitting a new shower mixer to an existing water system that currently has a Multipoint gas water heating system supplying it.
Firstly – if the current inlet water supply is running on “mains pressure” your hot water from the heating unit will also be at mains pressure. Because you are fitting a thermostatic valve does not nessessarily mean you will need to fit a pressure reducing valve to the cold side. Most new thermostatic shower valves are supplied as being suitable for mains pressure hot and cold.
The only time this would be a concern is if the pressure and volume of hot water is significatly less than that of the cold water at the outlet point. If this is the case then a pressure reducing valve would if fitted to the cold water inlet cause a reduction of cold water pressure and it could be manually balanced to be the equal of the hot water outlet pressure.
Another option would be to fit flow limiting valves to both or either of the inlets to allow for say 10 or 12 litres/minute through flow.
Hope this helps.
Selgas Services Ltd
Craftsman Gasfitters, Plumbers, Electrical Service Technicians