It is so very easy to get a pressurised hot water supply to a loft conversion without raising your cold water tank and from an existing gravity system if you can plan ahead. If the shower head or bath is below the water level of the cold tank you will be able to use the fully automatic SP21S (£155). If the bath or shower head is above the cold water level in the cold tank the automatic option is unlikely to work so you need the SP1 plus a SP1U (£140). All of these pumps are available from this web site https://showerpowerbooster.co.uk/shop/

Installation Steps
Turn off the cold feed valve which feeds the bottom of the hot water cylinder from the cold water tank.
Run an upstairs hot tap for up to 2 minutes after which time the hot flow should stop (the vent pipe is now empty).
Insert a compression tee into the vent pipe as shown.
Lay a new pipe from the tee to the shower or bath (22mm for a bath, 15mm for a shower)
Include the two ShowerPowerBoosters in the pipework (they fit to 22mm or 15mm pipes)
Add an isolation valve (optional)
Plug in the power pack into a standard 3 pin socket.
Add a radio remote if required (optional).
Turn on the cold feed valve which feeds the bottom of the hot water cylinder from the cold water tank.