Hello Ian,
Thank you for the background information in your post.
I am not entirely convinced the burial in 1833 is for your Mary Lawrence. The reason is that there was a couple by the name of Joseph and Mary Lawrence baptising children in Glastonbury at that time. They lived in High St and Joseph was a Land Surveyor. Judging from the names they used for their children, he was a son of John and Eleanor Lawrence. The Mary Lawrence who was buried could have been her.
I noticed that there were two ladies by the name of Mary Lawrence buried in Glastonbury in 1833, one aged 56 and from High St, the other aged 37. They are both shown as living in High St.
Thomas and Mary married in 1827, so I would have expected them to have been born in the early 1800s, up to around 1806. The details on the register do not indicate they were minors, aged under 21 at the time they married. Since the registers for Glastonbury are online, I would have expected to see Thomas's baptism on there. This suggests he was not born in Glastonbury, or that he was baptised in a non conformist chapel.
Looking at searches on Freereg, there seem to have been Lawrence, "colonies" in Somerset. Merriott was one of these. There were huge numbers of Lawrence baptisms etc there. There are some of Thomas Lawrence in the right period too.
My feeling is that Thomas was not born in Glastonbury but elsewhere. Working as a carpenter, you would expect him to remain in the same area and not travel too far away but you never know.
I will give this some more thought and investigation and see if I can find out more.
Jo