Hello, new on here.

I’m looking to get military costumes for a photoshoot. I’m based in Middlesex.

Does anyone know of any great places where you can try on and hire out costumes?

Probably any costume hire company would be able to provide a military costume of some kind. Surplus military clothing and reproductions of WW2 period uniforms are available from a number of websites. It would help if you could be a bit more specific about what you're looking for.

Not certain about the UK, but there's a military surplus store about 3 miles away from my house.
Camo gear, older military uniforms, belts, ribbons, gas masks, helmets, boots, and various other items (non-weapon).
I'd look around for a small strip mall shop.

Hope this helps.