The Bolt, Stock, Butt of the SMLE rifle is made of "Gun Iron" and is 5 inches long, 7/16 inch diameter and it is British Standard Whitworth threaded, 14 to the inch.
This is a 55-degree round-topped, round-bottomed thread.
You can START with a common 5-inch long by 7/16 National Coarse (also called UNified Coarse) bolt, but the threads will be a 60-degree SHARP V-thread.
You bring this American-size Bolt to correct British specification by modifying the threads by running a Whitworth DIE over them, re-forming the threads to the correct pattern. This will do. BSW and NC threads are the same PITCH, so this is already 14 to the inch, just a different form of thread.
BETTER would b to get a 5-inch unthreaded Bolt or a 6-inch threaded bolt, chop off the extra inch and make your own threads with the Whitworth die or a Lathe. The squared Tip and the Slot will have to be done separately.
BEST of all is finding a bucket filled with originals, in grease.
When installing, make sure you have a Stock Bolt Washer in place first. A very light coating of Grease prevents Rust.
The forged WAISTED Bolt was original during the Great War, straight bolts often being used interchangeably during refits. THREADS were held consistent on this part from LM through the end of production.
Hope this is a little help.