Apparently the answer is not easy to find. The UK missing Kids's database (http://uk.missingkids.com) lists 184 children (83 females, 101 males) that have gone missing in the last 20 years however it relies on accurate information from the police. 18 children went missing from the UK in the last year.
Kate Leaman, from Sky News, asked the same question back in 2088. In her article she quotes Lady Catherine Meyer, founder of Parents and Abducted Children Together.