You shgould contact the Society of Genealogists in London, they provide a number of different levels of course that will lead to a recognised qualification amongst the world of Genealogy. The courses are done as a correspondence course with exams set at a local level often via your local family history group.
it is important to distinguish the roles of professional genealogists and researchers. Genealogists direct the research and analyse information to produce an authentic Genealogy from primary and secondary source research.
It is that distinction that makes the genealogist. A Family Historian works with the larger picture and adds the bones to a family history rather than just the names and dates.
A record researcher is employed by a genealogist to carry out the practical task of the negative and positive research.
Becoming a member of the Society of Genbealogists would mean you have established yourself as a practising genealogist and can provide testimonials from clients. I would point out though, that being a paid genealogist is an extremely competitive area and it is worth advertising in Family Tree magazine for clients to give you a start on that path. Though again, taking someones money to do their family tree is only a profit making exercise if they understand the value of the work, You will have to be your own record researcher to get you established and then you may spend 60 hours looking for something and your client would not wish to pay for negative research.
http://www.sog.org.uk/acatalog/Society_of_Gene alogists_Publications___Products.html