So if self ID does come in, and I get a new birth certificate (with my new name and my sex marker changed), and other supporting docs, then for legal purposes I'd have changed my sex, and unless I volunteered this information, how would you know of any previous aliases as my paper history would be complete for my 'new' life, surely?
Correct, you wouldn't know. Some agencies keep records such as the police, DWP and HMRC but the files are only viewable by certain staff with special clearance.
(Does the above affect NI numbers too?)
This is from the general guidance:
www.gov.uk/government/publications/gender-recognition-certificates-t455
You should note that when HMRC are informed you are successful in obtaining a full GRC, they will update your record with any changes to personal details e.g. change of name and then they will restrict your record for specialist staff only to access; this is to comply with extra privacy requirements stipulated in the Gender Recognition Act 2004.