My wife and I are going to be in the UK for about 2 1/2 weeks.

Three weekends will be in London, where we will be staying at the In and Out club.

i'm gonna be going on a formal partridge shoot in the countryside about five days after we get there, so I need to know some affordable shopping options in London for a tweed jacket, shirt,tie  and breeks.

Also, any good military surplus shops?  Would like to pick up y'all's version of canteen,  cup, and stove.

James purdy and sons near Mayfair
I would google "army and navy shops" before you go to London.
Dont carry any blades longer than 2.5 inch
Atb
Mart

The store you are seeking old mate is Cordings of Piccadilly https://www.cordings.co.uk/us/our-stores/

Silvermans Surplus store in Mile End (east London) is not a bad place to look - https://www.silvermans.co.uk/

Enjoy your stay.