A few possibilities:
1. The MoD doesn't issue earplugs, or soldiers aren't hounded about using 'em. No idea on this one.
2. The Brits are fond of automatic fire from their SA80s. Possible that a Brit squad would expend a lot more ammo, thus making a lot more noise, than a comparable American squad. More noise = more chance of damaging your hearing.
3. Distance from ear to the front of the barrel is shorter than that of the M4 and M16. The overwhelming majority of sound, IME, comes from the front of the gun.
4. Wearing shorts damages your hearing.