On census day (30 march 1851), Thomas was living with Sarah, they claimed to be married but the evidence so far indicates They weren't.
At the same time Selina is a bar maid in London, living with uncle and aunt (James and Sarah Brunton) at 1 & 2 Peters Lane, parish of St Sepulchre, Borough of Finsbury. James Brunton is a victualler from Yorkshire. As Sarah's birthplace is Essex (looks like Rumset) she's more likely to be the direct relative. There are a lot of unmarried men in the household, no relationships given, but I guess it might be a lodgings house. If Thomas was in grocery / tea dealing trade, it's quite possible he did business with the uncle and met Selina then.
Thomas and Selina marry on Boxing day 1851 in her home village. They have a daughter Selina jnr early in 1853 (registered in Q1 1853) Hackney. Not a shotgun wedding, unless there is an earlier Burgis not registered in hackney or Essex, who didn't survive until 1861 census.
Thomas dies in 1856, is buried in Hackney on 4th Feb 1856 (the day before his birthday!). In 1861 Selina Burgis (widow) with Selina jnr is living with her parents in West Hanningfield.
There is a possibility that Sarah Burgis remarried in 1860. But if she had never been married to Thomas, if she went on to marry someone she would marry in her real name, not one she had assumed.