As you have already proven you are safe with the 22-250, a high velocity deer legal calibre, it would look absurd for them to place a supervision condition within your certificate to possess and use a larger deer legal calibre in the interests of "safety". If the argument is that you do not have enough experience shooting deer, and that is why a supervision condition is required, that again is nothing to do with public safety. In effect they would be conditioning the quarry species on the grounds of animal welfare which is not within their remit. The ACPO FELWG steer and new guidance is quite clear on this and a supervision condition within the certificate should be considered where the only alternative is likely to be refusal on the grounds of public safety.