Its mostly due to the hagus population. that and a combination of the ol' british overprotective nanny state. They figure velocity is the most important factor thus eliminating perfectly decent rnds from being used.
its really for your own good anyway, like the man said;
"If you ever have the good fortune to get tangled up with an hagus at close range in tight cover. I promise you the last thing on your mind will be "Well, Bell used a 7X57 mauser......"