Around 54,000 REME records were among the first WW2 service records to be completely catalogued by TNA and are found in WO420. In part this was beacuse the REME was only formed in 1943 and so involved fewer individuals than the other Regiments and corps where the records stretched back to 1921. Also done are the UK smaller Corps (RMP, RAPC, Intelligence Corps etc) whose records were originally held by the Bournemouth Combined Manning and Records Office. These are in WO421 with 94,000 records catalogued.
Work is in progress on the main bulk of records for the Infantry which can be found or will shortly be available in W422 and WO 423.
The work by Ancestry to digitise all these records is trailling behind somewhat as there is more work involved, and as Ross says Ancestry are only digitising the non-sensitive information and concentrating on attestations and the main part of the service record known as the Army Form B 103 Service and Casualty Form. TNA will generally hold additional undigitised records for most soldiers.