The will, made in 1788 before he set off on his last voyage.  (This is from an official copy that was made when it went through probate). 

And it says:

"In the Name of God    Amen 

 I, John Constant, now outward bound in the East India 

Ship Raymond, Capt. Henry Smedley Comm(ande)r. being in 

bodily health and of sound and disposing mind and 

memory and considering the perils and dangers of the 

seas and other uncertainties of this Transitory life do 

for avoiding controversies after my decease make publish

 and declare this my last will and testament in 

manner following, that is to say. First I commend my

soul to God that gave it and my body I commit to 

the Earth or Sea as it shall please God to order and..."

Well, you can see how it goes.  It's a bog standard will of the day - the bit about God is what everyone put in, and doesn't mean John Constant himself was religious (although of course he may have been).  But he, other Africans and indeed all people of colour were well advised to get themselves baptised, out of self interest.  At the time there existed a widespread, but unfortunately false, belief that a person who had been baptised a Christian could never be a slave.

And this brings up the great, great question mark hanging over the legal status of black people in Britain, who at the time might be classed as slaves elsewhere.   In Britain at this time they weren't...or not exactly.  The law was in two minds.  There were certainly hordes of British ships involved in slave-trading, and they quite legally took their human cargo from Africa to Britain's colonies, where the trafficked Africans were bought and enslaved by British plantation owners.  But that was all done under laws made in those colonies.  In Britain itself, their status was uncertain, because no legislation had ever formally been passed by Parliament defining who could, or could not be, a slave.

And somehow public opinion was not happy with the thought of slavery in Britain. Not happy at all.  Strangely, most British people had begun to feel that it might be all very well in foreign parts, and it might be a very fine, profitable and even glamorous thing to own slaves elsewhere - but not at home. It was not done at home.

In effect, this meant most black people in Britain were on much the same footing as the average white person, or else - very slightly better off.  They were seldom rich: but then neither was the average white subject of the King.  Most people were poor.  Most men - and all women - had no right to a vote.  The bulk of the British were not gentry or landowners, lawyers, shipowners, merchants or plantation owners: they were farm workers, spending their entire lives labouring on someone else's land for someone else's benefit.

But most black people in Britain were slightly better off than that, because most of them were household servants, and while 'servant' was perhaps not a great occupation it was in some ways an easier fate than being one of the rural poor: one of the English, Scottish, Welsh and Irish peasantry.  Unlike John Constant, most of them did not make a will, because most would never own enough to make it worthwhile.