Advertising & Design agency shortlist

I need a 'long list' of around 20 B2B/B2C advertising & design agencies. They should be independent - not part of an international group and not affiliated to an independent agency network. They should be an established business, 15 employees +, good clients, solid business. Geographically they should be biased to within easy travel of Windsor by train. Therefore, south-west/west London, Middlesex, east Berkshire and north Surrey. Would suit someone with knowledge of the advertising industry. Access to a list or database would help.