I note that the RN are "tucking in" then...
...also the 2011 DIB states quite clearly
Lightweight Jacket. A shirt that can be worn over a thermal or sweat-wicking layer. It can be worn either tucked in to trousers or loose depending on environmental conditions
That's that then, no more bollox about "it wasn't designed to be tucked in" - get it tucked in you scruffy herberts. Next you'll be wanting to grow mullets and beards to look like the SF
The B Word