I have no experience whatsoever with London. That said, however, I would imagine there are some universal practices when it comes to marketing one's home handyman services. Here are a few ideas:
1. See if there is a Web site that allows local businesses or companies to advertise their services. Here in the colonies, we have Craigslist. It's a nationwide posting service that is broken down by specific cities around the nation. So, if I have a business in Atlanta and want to market to local Atlanta residents, I can do so. There may be something similar in London.
2. Depending on your service, see if local merchants will allow you to advertise your services on their community bulletin board. Again, here in the Colonies, local hardware stores will allow plumbers, landscapers, carpenters and the like to promote their services to the community at large. This is a win-win, because it allows the store owner to offer the products and the services needed to install/use them.
3. Consider investing in a magnetic sign to place on your vehicle. This is a very inexpensive form of advertising. Since you will be driving your vehicle all over, potential customers will be able to see your ad everywhere. Be sure to print up some business cards or flyers to distribute, too, as you will undoubtedly have people asking for your phone number while you are out and about.
4. Along the same line as the magnetic sign, consider having your company name or logo printed on a hat or on t-shirts that you and/or your employees can wear while on the job. This will help create an air of professionalism about your business.
5. As for looking for actual clients, you might consider promoting yourself to others in related fields. For instance, if you build fences, you might consider contact local landscapers. Landscapers often time work closely with fence builders and deck builders on projects. Give some thought to which fields might be related to yours. In your case, you might want to reach out to remodeling companies, locksmiths, etc.
Good luck. I hope you find some of these thoughts useful. Even if they are coming from a Yank.