the chamber has its shoulder 1/8" further forward, due to headspacing on the case rim.
When developed in British Ordnance board decided that any picked up round would chamber and fire regardless of any debris / mud on that round.
So, when reloading, you'll have to "neck size" only and doing a full length resize will result is short case life, due to over-working the brass.
Kind of, sort of, but not really.
The .303 headspaces on the rim, true.
The shoulder shape & position is variable, true.
Neck (or partial full length) resizing improves case life, true.
It will however limit the rounds so processed to that exact rifles's chamber.
Not all Lee-Enfield chambers are that loose
, look at these 3 cases.
One is a factory unfired case (with the FMJ bullet)
One is a fired, but not resized case, showing how much blow out there is (or isn't) when firing.
The 3rd is partial full length resized with a SP bullet in place.