I'm aware of the current hallmarking legislation, but this item was made prior to 1975. Gemset pieces were exempt until 1975 ( I would say perhaps 95% of the prior to 1975 gemset rings etc. we sell in our shop have no hallmark, often just a stamp denoting the carat). Amendments were made to the 1975 Act in 2007, allowing items which could be proven to be manufactured before 1950 and of minimum fineness to be sold as gold without a hallmark. This was in recognition of the fact that so many older pieces were not fully hallmarked, so that they could be legally sold without the requirement to hallmark them before selling, as it was recognised that this practice was in some way defacing an antique item. If this locket is 1860's or thereabouts, these rules would apply.
I'd love to know whether a licence would be needed to retail this pearl.
....but, reconsidering your comments about the listing, and looking closely at the setting, it looks a bit messy, like the setting has been tightened on the pearl. Maybe it has been stuck in there as a replacement? To boost the price? Maybe it's a marriage? How, I'd love to be able to get a loupe on it!!