.303 is still listed as an eligible cartridge for UK 'Target Rifle' by our NRA (or it was a year or two back, although maybe things have changed now that ICFRA doesn't quote any metric military calibres any more, so that 7.62mm has become .308 Winchester and 5.56mm .223 Remington.) So in theory you could use a custom-built single-shot .303 TR rifle in the 'Imperial Meeting' at Bisley. The Catch-22 is that it's shot using issued factory ammunition, and the NRA doesn't obtain any of same. I was going to say nobody makes match quality .303" ammo any more, but that's incorrect as HPS-Target Rifle loads TargetMaster 0.303" and Kynamco Ltd also planned to produce its equivalent the last time I looked at its website, both using the 174gn SMK.
Historic Arms shooting is big in the UK and culminates in the annual 'Trafalgar Meeting' at Bisley each October, hence the demand for good factory ammunition. The Trafalgar includes several long-range matches up to I think 800yd, maybe even further. There are matches purely for Lee-Enfields in this and other Historic Arms meets, including some for the 'modern' versions with heavy 7.62mm barrels, and sniper variants.
If you get a good .303, it can be very rewarding to shoot with handloads, but the P'14 and the straight-pull Canadian Ross usually outshoot the Lees as do the better Mauser rifles like the 6.5 Swedes and 7mm Boers, also the 7.5X55mm Swiss jobs - hence the reason for L-E only matches. Getting a 'good' .303 is becoming problematical too as supplies of replacement barrels ran out years ago. Very few people if any know how to 'regulate' an SMLE's or No.4's bedding any more as was the practice pre-1968 when Service Rifle was the primary prone rifle discipline.
I would imagine that we'll follow US practice eventually on barrels, as with your commercial barrelmakers producing same-profile barrels for M14s and M1s if there is enough demand and the rules are changed to allow the practice.
I used to be very keen on all this stuff, but give me an F-Class rifle and little targets any time, especially as I see two backsight leaves one above the other, and can focus on the foresight blade OR the target, but not both together when I handle a military rifle now! Young men's sights!
Laurie,
York, England