The answer is St Pancras Old Churchyard in London.
This churchyard must have been the inspiration for his 1882 poem The Levelled Churchyard.
"O passenger, pray list and catch
Our sighs and piteous groans,
Half stifled in this jumbled patch
Of wrenched memorial stones!
"We late-lamented, resting here,
Are mixed to human jam,
And each to each exclaims in fear,
'I know not which I am!'
"The wicked people have annexed
The verses on the good;
A roaring drunkard sports the text
Teetotal Tommy should!
"Where we are huddled none can trace,
And if our names remain,
They pave some path or p-ing place
Where we have never lain!
"There's not a modest maiden elf
But dreads the final Trumpet,
Lest half of her should rise herself,
And half some local strumpet!
"From restorations of Thy fane,
From smoothings of Thy sward,
From zealous Churchmen's pick and plane
Deliver us O Lord! Amen!"
Hardy's reference to this churchyard was not the first by a notable author, in 1859 Charles Dickens used it in his 1859 book The Tale of Two Cities, where one of his characters was buried and another was known to have dabbled in body snatching.
NB Since writing this post it has come to my attention that The Hardy Tree has fallen due to bad storms in the December of 2020 - this is such sad news. You can read about it here: