I use Orbital Fasteners in Watford for 90% of my nuts, bolts, set screws, etc.
Prices are listed per 100, but they will sell any number of a given item, free next day delivery in and around the Greater London area (full list of qualifying postcodes on delivery charges page) for orders over £25.00.
Small online orders of less than £10 value and less than 1kg weight can be sent in a Jiffy bag via Royal Mail, (although not an option at checkout – see delivery charges page for full details).
Further afield, next day delivery is a little expensive at £60.00+VAT.