The following is taken from part III of Sir Thomas Erskine May's "Constitutional History of England since the Accession of George the Third", first published in 1861. The complete text of this classic work of "whig history" can be found here: volume 1, volume 2, volume 3.

Slavery in England

The free soil of England has, for ages, been relieved from the reproach of slavery. The ancient condition of villenage expired about the commencement of the seventeenth century; and no other form of slavery was recognised by our laws. In the colonies, however, it was legalised by statute (10 Will.III. c.26; 5 Geo.II. c.7; 32 Geo.II. c.31); and it was long before the rights of a colonial slave, in the mother country, were ascertained. Lord Holt, indeed, had pronounced an opinion that, "as soon as a negro comes into England, he becomes free;" and Mr. Justice Powell had affirmed that "the law takes no notice of a negro." But these just opinions were not confirmed by express adjudication until the celebrated case of James Sommersett in 1771. This negro having been brought to England by his owner, Mr. Stewart, left that gentleman's service, and refused to return to it. Mr. Stewart had him seized and placed in irons, on board a ship then lying in the Thames, and about to sail for Jamaica, where he intended to sell his mutinous slave. But while the negro was still lying on board, he was brought before the Court of King's Bench by habeas corpus. The question was now fully discussed, more particularly in a most learned and able argument by Mr. Hargrave; and at length, in June 1772, Lord Mansfield pronounced the opinion of the Court, that slavery in England was illegal, and that the negro must be set free.

It was a righteous judgment: but scarcely worthy of the extravagant commendation bestowed upon it, at that time and since. This boasted law, as declared by Lord Mansfield, was already recognised in France, Holland, and some other European countries; and as yet England had shown no symptoms of compassion for the negro beyond her own shores.

Slavery in Scotland

In Scotland, negro slaves continued to be sold as chattels, until late in the last century (on the 2nd May, 1722, an advertisement appeared in the "Edinburgh Evening Courant", announcing that a stolen negro had been found, who would be sold to pay expenses, unless claimed within two weeks). It was not until 1756, that the lawfulness of negro slavery was questioned. In that year, however, a negro who had been brought to Scotland claimed his liberty of his master, Robert Sheddan, who had put him on board ship to return to Virginia. But before his claim could be decided, the poor negro died. But for this sad incident, a Scotch court would first have had the credit of setting the negro free on British soil. Four years after the case of Sommersett, the law of Scotland was settled. Mr. Wedderburn had brought with him to Scotland, as his personal servant, a negro named Knight, who continued several years in his service, and married in that country. But, at length, he claimed his freedom. The sheriff being appealed to, held "that the state of slavery is not recognised by the laws of this kingdom." The case being brought before the Court of Session, it was adjudged that the master had no right to the negro's service, nor to send him out of the country without his consent.

The negro in Scotland was now assured of freedom: but, startling as it may sound, the slavery of native Scotchmen continued to he recognised, in that country, to the very end of last century. The colliers and salters were unquestionably slaves. They were bound to continue their service during their lives, were fixed to their places of employment, and sold with the works to which they belonged. So completely did the law of Scotland regard them as a distinct class, not entitled to the same liberties as their fellow-subjects, that they were excepted from the Scotch Habeas Corpus Act of 1701. Nor had their slavery the excuse of being a remnant of the ancient feudal state of villenage, which had expired before coal mines were yet worked in Scotland. But being paid high wages, and having peculiar skill, their employers had originally contrived to bind them to serve for a term of years, or for life; and such service at length became a recognised custom. In 1775 their condition attracted the notice of the legislature, and an act was passed for their relief (15 Geo.III. c.28). Its preamble stated that 'many colliers and salters are in a state of slavery and bondage;' and that their emancipation 'would remove the reproach of allowing such a state of servitude to exist in a free country.' But so deeply rooted was this hateful custom, that Parliament did not venture to condemn it as illegal. It was provided that colliers and salters commencing work after the 1st of July 1775, should not become slaves; and that those already in a state of slavery might obtain their freedom in seven years, if under twenty-one years of age; in ten years, if under thirty-five. To avail themselves of this enfranchisement, however, they were obliged to obtain a decree of the Sheriff's Court; and these poor ignorant slaves, generally in debt to their masters, were rarely in a condition to press their claims to freedom. Hence the act was practically inoperative. But at length, in 1799, their freedom was absolutely established by law (39 Geo.III. c.56).

The last vestige of slavery was now effaced from the soil of Britain: but not until the land had been resounding for years with outcries against the African slave trade. Seven years later that odious traffic was condemned; and at length colonial slavery itself,—so long encouraged and protected by the legislature,—gave way before the enlightened philanthropy of another generation.