The first thing you need to do is find out where the electric supply and water pipe enter the shower,this will limt the ones you can buy, unless the pipe or electrics can be altered easily, once you have found out this, have a look round, I think the best showers are Mira, but both the water and electrics come into the Mira showers on the right hand side, and I think it is the only make with this layout, decent makes are, Triton, Redring, Gainsborough, also remember to check cable size if it is a 6mm twin & earth, then don't get a bigger shower than an 8.5 KW, an electrician or plumber can advice you on ease of installation, good luck, Ray
PS, before you buy a new one have you taken off the shower head and given it a good clean ? and is the water pressure ok at taps, could be stop tap needs opening slightly to increase pressure.