This has been done for basically every other ex-British colony that has changed its name since independence, and I don't see why British Guiana is an exception. For examples of other countries in this boat, look at Zimbabwe and Belize.
I, too, fail to see a reason not to. We even have Demerara and Essequibo, so it would only be natural to continue the time line with a seperate British Guiana.
Indeed the country list is not consistent, we are currently working on fixing this. But a change in name or government is not enough to warrant a new country in Numista. The current guidelines say that there needs to be a significant change in territory for an issuer to be split, which is not the case for Guyana.
Demerara and Essequibo should indeed stay separate because they joined Berbice to form Guyana.