Anyone know if it is possible to use some form of mains pressure
>hot water cylinder that would not have its output flow rate capped
>by the mains inlet flow rate?
Only using advanced nuclear physics to convert energy into matter.
One other alternative is to use a tank and pumps. If you are planning a shower in the loft, this could be problematic due to the head being above the tank water level.
Then you have the choice of replacing your water main with something decent such as 25mm MDPE (assuming good street pressure), or using a pressurised storage tank, as would normally be found on a well system. This basically involves fitting a well type water system, but rather than putting it in a well, putting it in a small tank filled from your puny mains supply. The pump in the well system keeps the outlet water pressure at a nice high pressure and just tops the tank off slowly when the level drops too much.
Could be expensive, though.
Christian.