Suppliers

In no particular order except for the broad categories, a list of places that sell stuff that is relevant to our interests. Personal experience of these, good or bad, would be useful on the discussion page.

London retail or trade counter

• RS has most things, more professional. 4 trade counters, including Bow.
• Cricklewood Electronics A real shop, with little drawers of stuff & everything.
• Maplin chain, more hobbyist. Getting really poor at actual components.
• Cool Components Sparkfun stuff. Personal callers now allowed by appointment.

London-based, primarily mail order

(not known whether personal calling is allowed)

• Earthshine Arduinos and other parts
• skpeng

Mail order

• Farnell has everything, more professional. Owns CPC and Newark.
• Rapid mix of stuff. Colchester.
• tinker.it
• Amazon (next-day with Russ's Amazon Prime account) - equipment is often cheaper
• Bits Box
• Barend Hendriksen Holland. Interesting RF parts.
• Keytronics Obsolete IC supplier mail order, can also be found at radio rallies.

Classified ads and junk sales

• engineering machinery , materials and small ads
• amateur radio rallies have lots of interesting junk and random components

eBay

I have actual experience of these and received good exceptionally good service from them

• syringes (artag)
• soldering equipment (artag)

These sellers seem to have particularly interesting and relevant products, but I haven't necessarily dealt with them.

I have dealt with these sellers in the real world (and sometimes also on ebay) and will gladly use them again.