You can never have too many screwdrivers...flat, pozidriv, Phillips (yuk!) torx, torque, tiny wee ones for stupid wee dimmer modules, and a set of assorted sizes in a case with a spinner handle that takes a ratchet ( go to Halfords for the accessories for that...you'll find out!) plus a set of "security" bits, a tiny ratchet set with bits and sockets with some simple right-angle screwdrivers to get you out of trouble, plus get a Dremel gas blowtorch for heat shrinking where you have no power (it comes with numerous bits including a hot-air nozzle and deflector, so no naked flame) and a 50m wander lead and a set of walkie-talkies if you work with others, and of course a 40W site radio to charge your batteries and annoy the ---- out of everyone else on site...and if you haven't got any ferrules and a decent crimper, add that to your list, insulated and uninsulated ferrules, and one of those machines that let you take card payments on the job, a big bargain box of TurboGold screws, a Bosch Go electric screwdriver for faceplates and general screwing (!)
numerous head torches and portable lights, decent cutters/shears and a pair of installation pliers...and finally, don't laugh...
A really cute Labrador, well trained...the ladies love 'em!
Hope this helps!