Ignoring all of the pointless and rude postings, the facts here are:
You can replace the shower with another 7.2kW model without changing anything else.
To use a shower of higher rating, the circuit breaker will have to be changed, and it is very likely the cable will have to be replaced as well.
The control knob on electric showers alters the flow of water. Less water flow = hotter water. The power used is the same.
Some models have a 'half' power switch which does use less power. However you can't fit a shower relying on the fact that it will only be used on that setting.
'mechanically protected' refers to wires or cables contained in something like steel trunking or conduit.
Supplementary bonding is explained
here
Your choices are:
1. Get another 7.2kW shower and fit that.
2. Find an electrician (search here:
http://www.competentperson.co.uk/ )to fit a shower of higher rating, upgrade the circuit breaker and cables as required.
Given that you have already said this:
Look, pal, I'm not an electrician. I've never so much as changed a fuse!
the third option of notifying your local building control, paying their fee (£100+), installing the shower, cable and circuit breaker etc. yourself
is not going to be possible.