Ammo back when the LM was the standard rifle was the Black Powder Mark I or II, Cordite Mark I or II. The Mark VI was a post-Boer War re-creation of this load. As far as I can tell, the Mark VI Cordite load went out of production in Canada in 1911, the others much earlier.
All had a 215-grain bullet, round-nosed and FLATbased. The closest relatively-inexpensive bullet today would be the Hornady 174 RNFB.
Cordite back in those days was extremely energetic and extremely erosive. It utterly ATE the Leades out of barrels..... and did it in jig time, too.
The problem now arises that you will NEED a relatively-fast powder because it is entirely likely that even the soft Hornady bullets are going to need a sound kick on the backside in order to set-up and seal your bore.
Might I suggest 13 or 14 grains of Red Dot behind a flatbased CAST bullet, gas-checked, of course? LEE has a mould which will turn out this bullet, a 184, for about a penny a pop, given that you have some old wheelweights lying around. The gas-check costs you another 3 cents, the powder another 6 cents. You end up with 8 rounds for a buck.
That puts you in business with a mild load which ought to seal the bore and even produce half-decent accuracy with just a bit of luck. If it does not quite seal the bore, go another half a grain of powder, but I would not exceed 15 by any means.
And a nice point is that that shallow-rifled 7-groove Metford barrel will last about 5/8 of forever.
Good luck.