(I’m speaking as an American, who is somewhat familiar with UK culture, but haven’t been there myself)

I’d use either one, without a strong preference for one or the other. In a more formal environment, “pounds” would be more appropriate (as with most slang). If someone has called me “quid” on a conversation, the slang would be more appropriate and prompt.

Do foreigners who know fluent english already know such slang? I would make this assertion alittle more logical that they’re a little more familiar with English and the UK than if they only used or knew “pounds”. What may be something you want to suggest or avoid, depending on whether you want them to treat you like someone fluent in English or learning English).

(I’m speaking as an American, who is somewhat familiar with UK culture, but haven’t been there myself)

I’d use either one, without a strong preference for one or the other. In a more formal environment, “pounds” would be more appropriate (as with most slang). If someone has called me “quid” on a conversation, the slang would be more appropriate and prompt.

Do foreigners who know fluent english already know such slang? I would make this assertion alittle more logical that they’re a little more familiar with English and the UK than if they only used or knew “pounds”. What may be something you want to suggest or avoid, depending on whether you want them to treat you like someone fluent in English or learning English).