daffy is quite correct, GP's have "all on" to deal with the sick and giving M.O.T's to the under 45's would be impossible.
What you are suggesting a "Well Person Clinic" is a good idea in theory, but expensive in practice and no country could run afford to run such a clinic.
Yes, ummmm you may pick up a symptomless disorder, but it would be so unusual that the cost would be prohibitive for such a scheme.
Go to your garage and suggest a FREE M.O.T on your care.
B.U.P.A advertises a complete check-up, but you have to pay.