Military service records are not subject to any ‘100 year’ rule, WW1 records, such as have survived, have been available on Ancestry for several years.
WW2 service records can be purchased from the MOD, here is their web site
https://www.gov.uk/get-copy-military-service-records
Note the 3rd bullet point below!!! The cost is £30 none refundable, even if they do not find anything. Good hunting.
<h1 class=”part-title” style=”margin: 20px 0px; padding: 0px; vertical-align: baseline; border: none; font-size: 27px; font-family: nta, Arial, sans-serif; line-height: 1.11111; color: #0b0c0c;”>Apply for the records of someone who’s deceased</h1>
<div class=”govuk-govspeak direction-ltr rich-govspeak disable-youtube” style=”margin: 0px; padding: 0px; vertical-align: baseline; border: none; color: #0b0c0c; font-family: nta, Arial, sans-serif;”>
<p style=”border: none; margin: 0px 0px 20px; padding: 0px; font-size: 19px; line-height: 1.31579;”>You can apply for a copy of someone else’s service records if any of the following apply:</p>
<ul style=”list-style-position: outside; list-style-image: initial; border: none; margin: 20px 0px 20px 20px; padding: 0px; font-size: 19px; line-height: 1.31579;”>
<li style=”border: none; margin: 0px 0px 5px; padding: 0px;”>you’re their immediate next of kin, for example their spouse or parent
<li style=”border: none; margin: 0px 0px 5px; padding: 0px;”>you’ve got consent from their immediate next of kin
<li style=”border: none; margin: 0px 0px 5px; padding: 0px;”>you have a general research interest – you’ll only have access to limited information, unless they died more than 25 years ago
<p style=”border: none; margin: 20px 0px; padding: 0px; font-size: 19px; line-height: 1.31579;”>You need to know the person’s full name, date of birth and service number.</p>
<p style=”border: none; margin: 20px 0px; padding: 0px; font-size: 19px; line-height: 1.31579;”>Fill in 2 forms – a request form and a search form.</p>
</div>