From water tank to combi boiler and need to replace shower

It’s a combi boiler so it’s mains fed and pressurised, so assuming the shower is not a million miles away from the boiler then you can pretty much have what you like. The flow rates would need to be checked first though 👍🏻

You need a Thermostatic shower as your water supply is mains fed now

You can have most of the showers as the pressure for hot water that comes from combi boiler is in 70% the same as your mains cold pressure. The only think is avoid using service valves with a small bore hole as well as make the plumbing for it as easy as possible without any flexi pipes or crazy amount of elbows. After installation of the shower and tests its a good practice to remove the shower and clean the washer filters as sometimes some debris from plaster, tiling or something else can interrupt the volume of the water. Please also check if you stop cock valve is completely opened (open it until you will feel there is an end and do half turn clock wise to avoid damaging the glens).