MBNA use to have a maximum limit on their Platinum Plus cards (Could be others as well) of £50,000. Don't know if they still do as their website doesn't state it anymore.
So, if someone does get a limit of over £25,000 does it mean that they are not covered by the CCA? I assume this would be legal? Do they offer this (high) limit because they are an American company and possibly the CCA doesn't apply to them (ie because they are based in the US) or are there other (UK) credit cards that offer/extend a limit higher than 25k?