The United Kingdom reserves no right to make amendments. Individual foreign awards for gallantry or meritorious service follow the same staffing route. Both may be subject to scrutiny within FCO which must take account of the employment of Service personnel as State Servants, although it is usual that life saving actions may be recognised by the appropriate Head of State. No campaign medal for wearing, UK or foreign, may be claimed through concurrent service which gains another campaign medal. In that case, Service personnel must excecise judgement as to whether refusal might give offence.
Good manners are the key and usually a gracious acceptance is the proper action.
Thereafter the Chain of Command is to be informed together with any request to wear and a justification. Usually the recipient will be allowed to keep the medal as a keepsake.