Brace position - Why hands atop head in the UK, but on ankles or atop seat in front in the US?
The difference is just because there are often disputed methods on which technique is best. To be brutally honest, many accidents are so severe and can't really be analysed, so this is still a point of contention.

I would hypothesize that the UK version is safer as the hand position protects the skull and face from direct impact with the seat in front; instead colliding with the inside forearm. This will naturally reduce impulse (strengthy plastic vs. fleshy skin) and should thus reduce injuries.

The concept of holding ankles stems from the position your body is put in when holding your ankles. Any kind of "holding" is fine, touching on top or sides. It puts your body into a position that ducks your head and should (because of close seat proximity) rest your head against the seat infront anyway.