The latter is not possible as meters will be set at the point of manufacture. I believe you can buy SD Codefree monitors in the UK using either scale, so you could see about buying a new one, or if you’ve just bought one and not used it they might exchange it for you.
Thank you for your answer, it is a new meter, however having read where I bought it from it is already supposed to be in mmol/L and so I'm sending it back