Extending electric wiring

Ok, I posted a while back about getting my loft insulated. The bods who have come so far could just as well asked me the measurements and saved themselves the journey, advice nil. It is obvious that among other things I will need to spend a lot of time "extending" the existing mains wiring so that it will be above the insulation. If I judiciously cut wires and fit appropriate junction boxes will this be ok? If so that leaves the armoured cable that runs from the garage (where the elect-trickery originates) the length of the loft into the mains distribution box. Will I need to deal with this or will that be ok covered? I will also need to extend the beams as access is needed as the central heating pipes also run through the loft. Obviously I need to wait for the temperatures to drop, I don't fancy heat exhaustion. al advice welcome, though not necessarily heeded! :-)