IMO there are a few possible problems. Raising the height will increase the pressure but how high is it now and how high can you make? Each meter is 0.1 bar so if you are only moving it up say 0.3 then it might not be worth it. Secondly is the loft tank running dry or low when you have the flow issues? If level is dropping a lot then that won't help. A bigger tank would be one solution but you could also look at a higher flow ballcock to replace the water faster. Finally do you have the right sort of shower? If you have fitted a shower which is only good for 1bar or above then raising the tank won't do a lot (1m = 0.1bar so you need 10m for 1bar!). Only solution is something like an aqualisa colt or a shower pump.
