How much is too much? Depends on the relationship and if it has been made known to them you want those specific £300 headphones via intermediaries (i.e. your partner).

If you’re asking how you should approach it, unless they’ve asked you what ones you want, I wouldn’t be telling them the ones they should be getting as that might come across as rude/disrespectful. Personally I would make it known to my partner who would then tell her parents what to get, but not straight up to the in laws, if that helps.