Off the top of me head, in addition to accessories/cable already mentioned I keep: a 3m tail kit, a couple of henleys, REC, 12V 0-150W transformer, BS951 clamps. A couple of stanley cases with various bits and bobs: heatshrink, soldering iron/solder, various cable clips, cable ties, rawl plugs (mostly brown/red (but keep a few blue/yellow as well)), grommets, various lengths of 3.5mm machine screws, spare stanley blades, a box of PZ2 bits. Also mixed pack of resistors, electrolytic capacitors. Also capping and some trunking (tend to buy bulk packs and keep what's left on the van, it's stored in an bit of 100mm soil stack (not used!)). Oodles of sleeving (gy(5mm/3mm)/brown(3mm)). Various sizes of fuses (and fusewire). I also have a case with various screw sizes (orig screwfix). A few double plasterboard boxes, a few surface mount boxes (single and double), metal boxes ... not huge quantities just a few of each ... saves having to nip to the nearest outlet on occasion.
I also keep singles and a few 20mm conduit lengths, bends, sockets and glands (and 2.5mm brown/blue/gy). Metalclad sockets/switches. 2.5mm T/E, 1.5mm T/E (usually have some 6mm and 10mm T/E left over, but don't "intend" to stock it).
Used to keep 60W/100W incandescents and GU10's, but don't any longer (to much choice on type of lamps these days).