Yep need to know the rating of your wiring and its worth knowing how much pressure you get in the place you want to put the shower.
If theres not a lot its worth looking at getting some sort of pump which is a more sensible option than just getting a higher kw rated shower....
Mira and some brand beginning with T are pretty popular on the shower front.
If all else fails ring a plumber or have an OCUK meet at your house with about 30 of us trying to fix it in one afternoon. Actually just have the meet there a week saturday, if enough of us turn up we can fix your shower and end up starting several other DIY projects for you...