The amount of water you get out depends on the pressure pushing it along the pipe combined with the effective cross-sectional area of the pipe. As per the previous reply, part of the problem may be caused by furring up, which reduces the effective cross-sectional area. If you have a cold water tank in the loft somewhere feeding the hot water system the pressure is determined by the 'head' of water - or the vertical height between the tap outlet and the level of the water in the cold water tank. If you have noticeably more hot water emerging downstairs than up, then the head is probably inadequate. You may be able to increase it by raising the water tank on wooden blocks, or fitting an electric pump. Both probably jobs for a plumber if you haven't tackled such a thing. If it is furring, it will occur in the hot water tank and is likely to impact all the hot water outlets - not just the bath. You really need a plumber advice, then get back on this website for further info before you decide what to do - he may try and sell you an unnecessary solution.