Service records post 1920 are held by the Ministry of Defence or at The National Archives. As records are moved to The National Archives check there before going to the MOD, so you won't find them on Ancestry or any other site. You will need to apply for the records - a death certificate, date of birth OR service number, a cheque for £30 and a lot of patience, as the waiting list is over 12 months

https://www.gov.uk/guidance/request-records-of-deceased-service-personnel?fbclid=IwAR1G4HyxadB0cq_EpYOR9Xw9oixOrthVVHq9vfo2-pCF4wzPGS6M4558e98

The National Archives - Now have some records

Second World War – an overview

Their guide provides a brief overview of Second World War records held at The National Archives. These are the records of central government, including all branches of the military. You’ll get a sense of the sort of records we hold and links to guides with more details and advice on how to find the records in our collection, including whether records are available online.

The original records are arranged at The National Archives by the government departments that created them. Each department’s collection of records is identified by a department code. Three of the most significant for Second World War research are:

• the War Office, which administered the British Army – department code WO

• the Admiralty, which administered the Royal Navy – department code ADM

• the Air Ministry, which administered the Royal Air Force – department code AIR

Numerous other government departments have transferred Second World War records to The National Archives. Search the website to find out more.