"Will it have any detremental affect if I use the 2.5" well as to working no it will still work but would not comply with regulations a change in cable size can only be done in some form of distribution unit as in the future someone may see the 2.5 cable and think they can upgrade other bits to match.
"what amp connector blocks should I use?" non connector blocks can only be used inside a junction box and this must be accessible and in the main the connections are all part of the junction box and not a connector block.
Remember all tests must also be completed and prospective volt drop, earth loop impedance etc. all entered in the minor works certificate. This will be required for insurance. There seems to be a little confusion as to if this comes under Part P so I would follow links and make your own mind up. Also alterations to lights is by far the biggest reason for people asking for help. Read the projects first and take special care with black or blue wires. Never work live and if your not sure you know how don't do it.
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