I get the use of local wholesalers of course, and indeed use screwfix myself, but I would have thought for planned CU replace or rewires, most would buy in semi-bulk, certainly if I actually do a job (rather than poring over designs all day) I tend to order most everything from chosen suppliers (CEF, Edwards, consumer unit world etc) mainly on-line, and it just gets delivered. (Screwfix good for cable though!) - allows me to get the ordering done in the evening and work in the day.
I don't really do fixes/faults, but in that case I would just use CEF, Yesss, or screwfix/toolstation, depending on the needs