While many travel agents receive the bulk of their business through referrals; many people are turning towards the internet still in search of a good local travel agent.
Here is a brief list of some of the more popular travel agents in the UK. I've also included a brief description highlighting their differences (corporate versus private, flights to US vs. domestic, etc.)
http://www.kuoni.co.uk: If you're looking for long-term travel.
http://www.travelcounsellors.com: One of the UK's largest travel agencies. Highly recommended.
http://www.waytetravel.co.uk: Specialists in corporate travel; great business class fares.
http://www.jetsave.co.uk: Great fares to North America.