S&K make a no-gunsmith mount for the SMLE (Number 1 Rifle).
I used to get them through MilArm in Edmonchuk.
About $125 but very solid and puts the scope right over the bolt: where you need it.
You don't need a great deal of scope with a .303 because the performance is relatively mild, trajectory starts to get rainbowlike after 400 yards or so. For 300, sight her in for 225, raise or lower what you need. The 150 bullet in a .303 runs round 2700 or so, which actually is pretty close to .308, so that gives you an idea about drop. The 180s run about 2440, which is MilSpec.
Hope this helps.