The Scottish requirements for bullets used to shoot roe deer are entirely logical.
Each of the 3 factors is a SEPARATE MINIMUM requirement, with any combination of bullet weight and muzzle velocity required to make the minimum muzzle energy
A 50 grain bullet does need an MV of more than 2450fps to make 1000ft lb but, for example an 80 grain bullet with an MV of 2450fps will have a muzzle energy of more than 1000ftlbs grain bullet and be perfectly legal.
Essentially, placing the limits on the projectile rather than the calibre gives shooters many more potential options as to the calibre of the rifle they can use to shoot roe deer
The debate in the House of Lords back in 1985 that lead directly to the Scottish legislation is worth reading
Hansard record of the item : 'Deer (Firearms Etc) (Scotland) Order 1985' on Wednesday 10 July 1985.
hansard.parliament.uk
The bullet weight/ MV/muzzle energy numbers were not picked out of thin air by politicians - they came directly from the Red Deer Commission - a body that knew quite a bit about shooting deer.
That bit of legislation has put Scotland in a far better situation regarding the shooting of roe deer by removing calibre from the legislation and concentrating on the projectile and it's ability to kill the animal humanely.
I think it's fair to say that in Scotland more roe deer are shot with .222, 223 and 22-250 than anything else.
They all do the job very well, but would be illegal to use on roe deer outwith Scotland
Cheers
Bruce