There is no equivalent to the UKM registration process for those born before 1983 with a mother who became a British overseas territories citizen. If you look at the UKM criteria, one of them is that you would have had a Right of Abode in the U.K. immediately before 1983 if you were a U.K. and Colonies citizen at that point. Unless you have a U.K. born parent, grandparent (paternal or maternal), lived in the U.K. for some years prior to 1983, or were a CUKC woman married to a man with right of abode, this is unlikely.
If you were born in Antigua before independence- having a mother connected with Montserrat would not in general have prevented you from losing British nationality on independence, if you became an Antigua citizen. At the time- the exception to loss was based on links through the father, not the mother. In the case of Antigua, there was also an exception for those with Right of Abode.
https://www.gov.uk/government/uploads/s ... ndence.pdf
https://www.gov.uk/government/uploads/s ... toloss.pdf
Any siblings you have born in 1983 or later may have a good chance of claiming both British overseas territories citizenship and British citizenship by descent, however it's not clear what pathway there currently is for you. It is always possible that law/policy may change in the future so you should stay in touch with developments.
This is not intended to be legal or professional advice in any jurisdiction.