Roughly I keep:
1 box of:
1G sockets
2G sockets
FCU's all flavours
blank plates
pendants
battens
20A switches all flavours
Light switches all flavours
And then...
50A 2G switch
45/50A 1G switch
Cooker control switch
A few lamps
back boxes - all flavours and sizes
henley blocks
MCB/RCBO's commonly used all ratings usually type B's and C's
CU parts commonly fitted
CU glands
Grommets blind, open etc
25mm and 16mm 3m tails
16mm earth
10mm earth
6mm earth
1.5 and 2.5 singles brown, blue, green
8 way MET's
Din rail
1.5, 2.5,4,6,10mm T&E
1.5 3 core
flex HR 1/5,2.5
5 core flex
3m of h05
A few rods and accessories
grommets all sizes
accessory screws all lengths
brass screws
BZP screws
stainless screws
Wages all sizes
Wago boxes
Wiska boxes
Line boxes
IP65 glands
cables clips all sizes for flex and T&E
metal cable clips
wall plugs various varieties
Crimps and heat shrink - various
Few bits of conduit accessories (main box kept elsewhere)
tube of conduit various
Tube of trunking various
Hope that helps?