You should go to the Association of Buisness Angels. However, watch out!, they will probably try and charge you £500+ to write the business plan in the format that their investors like and expect.
In my limited experience of dealing with business angels, you may be better off networking and finding a high net worth individual to back you initially. Not only is this cheaper - it is often quicker. Although I am sure many people do have success with business angels.
Alternatively you could go to a corporate finance broker. However, it is unlikely that you will get one to produce a business plan and market it to potential investors for less than £1000+ upfront plus a large success fee (ie £20,000+) if they succeed in raising finance for you. A good broker will charge more.
Be suspicious of anyone offering to work for you for free or on a 100% success fee basis.