This is another confusingly–worded question, IMHO.
It's asking for the UK's largest lake (by surface area). So why doesn't it say that?
By using the phrase "body of water" instead of "lake", the question setter is potentially sowing seeds of doubt in the contestant's mind. Does he (or she) mean, for example, the Bristol Channel? Or the North Channel, which separates Northern Ireland from Scotland? Or Morecambe Bay, perhaps? Is Scapa Flow bigger than Lough Neagh? (I've just checked by the way, and it isn't.)
Are any of these bodies of water actually in the UK? Probably not (with the possible exception of Scapa Flow). But as I said: the use of the phrase "body of water" is potentially enough to sow seeds of doubt in the contestant's mind.
I'm guessing that the setter avoided using the word "lake" because Lough Neagh is called a Lough, not a Lake. But this to me is an irrelevance; the common or garden English word used for a body of water such as Lough Neagh is 'lake'. Just because a body of water has a name that's derived from a language other than English, doesn't mean it's not a lake.
Wikipedia seems to agree with me: Lough Neagh is, it says, "the largest lake by area in the British Isles".
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