The surgical method used in days of old was positively barbaric Lord Flynn. It involved tying the cockerel down and hanging weights off the legs to stretch it out, then without anaesthetic the chest cavity was entered to remove the testese which are situated between the wings. The survival rate at 6 weeks old was about 75% I read! it was also rather time consuming. Can't think for a moment it would be legal.
As far as I know hormonal caponisation is practiced in France, where I presume it is legal, but can't be certain. I do know the bird is supposed to get even more tender with age, but there will be a point at which extra feed results in insufficient growth to warrant continuing.