Windsor Davies wants his shirt back...My main criterion is a button-down breast pocket on each side, so I can I put my phone in one and my wallet in the other and not be all lopsided!
My lightweight summer shirt is an Army Navy stores one I bought at least 20 years ago. Still in good condition.
My heavier cotton twill one came from Primark of all places. You'd never know it wasn't a specific countrywear type garment. Had it a few years now - very sturdy and robust. The make is Farrell. This is not identical, but it is quite similar...