It depends what you have supplying your exisisting shower, hopefully you have a 6mm twin and earth on a 32Amp breaker, if so then an 8.5 will be fine.
Feel free to take a picture if your unsure of cable size.
If you fit a decent sized combi it will give you all the hot water you need, combi's are all to do with flow rate! don't go for your entry level 24kw! Smash a 42 kw Worcester in!
These are the words according to Butch lol