I had my mare freeze-marked and she was an utter nightmare, so get some acp not sure about new name from vet and I went with Farmkey as with my other horses.
Farmkey are the best in my opinion. All my horses have been freezemarked by them, the most recent being last Tuesday. Very happy with the service, competent and professional in their approach. Good luck, its a good visual deterent from would be thieves.
I have used both of these companies this year and found both to be competant so i dont think it matters to be honest.
Although i did use farmkey first and rang them again to do one more they had a long waiting list and freezemark came out 13 days after i rang them i think i was just lucky that they had a booking in my area so soon.