changing mixer shower to Power shower

Hi, I have bought a new power shower (Aqualisa) and want to fit it in place of my existing mixer, I have a gravity fed system with my cold supply to the mixer teeing off the cold water feed to the hot water cylinder, the hot to the mixer is then taken from the top of the cylinder, I don't know without ripping all the floor up whether its the first feed. All diagrams I have looked at show a totally separate feed for the cold water from the cistern in the loft and a hot water feed first tee off from the top of the cylinder with stop taps on each. Can I just use the existing connections from the mixer into my new unit (I guess this is a balanced system so unless there are regulations that say no it would work)? Are there any regulations that say I must make new tapins for each supply and put on the stop valves? My property is 10 years old so I guess things should have been done right for me to just connect up but thought it was worth checking with you experts.

By the way I am in the UK

Your help is much appreciated.

The reason for separate feeds or first feed is to stop variation in water temp when other taps or draw offs are used,so you could use the existing pipes and it could work fine,but it might cause temp/mixing problems. also would invalidate warranty if not installed to makers instructions.