People registered under the UKM route (under
Section 4C of the BNA 1981) will be
British citizens by descent and therefore their children born abroad will not automatically acquire British citizenship.
For the children to acquire British citizenship, they will need to be registered as British citizens under
Section 3(2) - if the British citizen parent (registered using Form UKM) has resided in the UK for at least three continuous years before the birth of the child - or Section 3(5), which requires the child and both parents to have resided in the UK for at least three continuous years before the child's 18th birthday.
Note that Section 3(2) registration also has requirements of the child's grandparents and therefore it may not be applicable to children of people who registered via the Romein route.
Of course, children born in the UK after the parent registered using Form UKM are automatically British citizens otherwise than by descent in their own right.
I am not a lawyer or immigration advisor. My statements/comments do not constitute legal advice. E&OE. Please do not PM me for advice.