Hi Loz,
I understand your feelings about the fire proof downlighters - if you feel more secure fitting them then this is worth doing. Should the worst happen and you have a fire in one area, if they buy you that extra bit of time to get you and your family out then they can only be a good thing. I would have gone down this route myself but i wanted the square type made by Aurora, and unfortunately these aren't available fire rated.
I have just dug a set of instructions out from the Click range and they say "This fitting requires a distance of 50mm of free air around the sides and a distance of 20mm of free air above it" so you will still need something to keep the insulation from resting on it. If you have a few in a line, maybe this could be achieved by fixing two wooden battens across your joists 50mm from each side of your fittings? this way the insulation will lay across these with at least 50mm space all around if you use 50 x 50 battens.
As for what you are allowed to do yourself under Part P:
Remember that the IEE regs and Part P are completely different entities. If you do any work yourself, if it is done properly it will comply with BS7671. Part P of the building regs however (in a nutshell) does not allow any alterations or additions to fixed wiring without the proper certification by a 'competent person'.
Basically, if you change a downlighter, as long as you do not alter the fixed wiring i.e. the twin and earth, this will only constitute a fitting change which you are allowed.
If you want to add lighting points / circuits / switching arrangements etc then this work will need to be certificated to comply with Part P. Your official options here are:
1. Get a Part P approved electrician to do the lot and leave you with a certificate.
2. Do the work yourself and get the work tested and inspected by a Part P approved electrician
3. Do the work yourself and have it inspected by your local building control (this will need to be done twice - once at first fix, then at 2nd / final fix).
Your unofficial option (which i am not suggesting you do) is to do the work yourself and say nothing. After all, who is going to come and check? This is the problem with Part P - it has become expensive and complicated to the consumer.
Part P is a bit of a shambles. It was a good idea in that the aim was to get rid of cowboys in the way that Corgi helped the gas fitters, however its execution has left many problems. It is now possible for any Joe from a semi-electrical / diy background to pay his 700 quid, do the minimum to pass the course and be regarded by the public as a safer choice than a genuine electrician who up until now could issue a certificate in his own name.
I have been in the electrical contracting trade for 13 years within an NIC EIC and Part P approved company and studying for 10 of those to reach my present position. Unfortunately, I am regarded as no longer safe to do any electrical work independantly, but kitchen fitter Joe is.
Anyway, rant over, hope this helps