I have been using Wolf steel case 303 while restoring my MKI Bren. Barrel is a MKI with correct headspace and no issues with the steel cased ammo once I got my gun running correctly.

I am down to maybe 80 rounds left and none to be found.
That will steer me to my +500 rounds of HXP brass and reloading if I can't find any cost reasonable live ammo.

Buddy has some boxer primed Canadian surplus 303 that seems to shoot good around $1 round. This is ball powder loaded, NOT Cordite.

PPU is also popping up around $1 a round and non corrosive and boxer primed.