Ancestry adds UK, Military Indexes, 1920-1971

Ancestry describes these lists as comprising "the names and service numbers of those who were discharged from the armed forces after 1920 and born before 1901. Details given for over 300,000 (371,716) individuals found within this collection may include (where available):

Initial and Surname
Date of Birth
Their Service
Service Number
Ministry of Defence Reference Number."

It's most likely to be useful for the date of birth information.

The source, "public sector information licensed under the Open Government Licence v3.0," leads to a collections of datasets hosted by the UK National Archives. I found little of genealogical interest, many are merely catalogue entries for unpublished datasets.