There were cases where slaves who had been imported into Britain left their 'masters' and eventually gained freedom. However, it would be massively difficult for a slave to get to Britain from overseas, unless they were part of a mutiny that took over a ship. Such mutinies did occur (like the famous Amistad incident), but I am not aware of any of those ships sailing to England. Often the slave mutineers were captured by a military ship and were subject to maritme law (prison or hanging) or at least returned to their owners. The more likely scenario would be to escape to lawless zones in the Caribbean islands or Brazil that had enclaves of escaped, free slaves.

England in the first half of the 19th Century did patrol the African coast to suppress the international Slave trade. Their success was limited however. For a brief time even the US sent a few ships to the African coast to patrol (the US having outlawed the slave trade, but not domestic slavery). When a slave ship was captured the slaves would be brought to a free port in Africa, such as Sierra Leone.