Although TW is mainly correct, some blacks were brought to the UK at the time of the slave trade, but not as slaves. They were usually employed on the staff of large houses(although I don't know if they were actually paid as were white people in service) and used as footmen etc, They were usually 'used' more for their oddity value as much as anything, because of course at that time hardly anyone in the UK had seen a black person. They were usually referred to as 'blackamoors'.
There was a very interesting programme on Radio 4 about very small black children being brought to England to be pages for fine ladies. The children were generally treated very well and were dressed as finely as their ladies.The usual stipulation was that they be as black as possible - in order to emphasise the fairness of the ladys skin. Sometimes they were fitted out with silver collars - in the same way as lap dogs were.
Wait a minute, Tartanwizard! What do you think people of African origin were doing in the West Indies in the first place? They were, of course, transported there as slaves in the 17th and 18th centuries, mainly to work on sugar plantations. Only in 1834 did the UK abolish slavery in the West Indies, well after it had been abolished in the northern states of the USA.
Without going into it all the Romans had legions composed of black people and some of these legions were stationed in Britain. Many of these men fell for the local girls and opted to be de-mobbed in Britain so that a great number of us will unknowingly have a black man in our ancestry.