According to the BTO, the most common bird in the UK is the wren, with an estimated 11 million breeding pairs, while house sparrows only rank third with about 5 million.
The cuckoo clue is just bad. Sure, on rare occasions a cuckoo may feed on eggs or chicks of other species, but most of the time, it just lays its egg in the nest and the freshly hatched cuckoo chick will push the host species' eggs out of the nest. There are other species that are much more frequent nest raiders.
Some of these are a little contentious. I would have thought that goldcrests and firecrests are both smaller than wrens. Capercaillie is listed as Least Concern; there are several other species that live in the Scottish Highlands and aren't doing too well. If house sparrow is the correct answer then sparrow should probably suffice. Sanderling is in the Calidris genus of stints and sandpipers and isn't closely related to curlews. Temminck's Stint is the only other bird with stint in its name, not the only other stint. Not sure red-footed falcon counts as a UK bird even if you may get the odd vagrant. Lots of answers to nest raiding question. Maybe put "true" before aquatic songbird.
Sometimes quizzes make me realise I've been misreading/misspelling a word for years. This time it's the Temminck's Stint (always thought it was Temmick's!)
The cuckoo clue is just bad. Sure, on rare occasions a cuckoo may feed on eggs or chicks of other species, but most of the time, it just lays its egg in the nest and the freshly hatched cuckoo chick will push the host species' eggs out of the nest. There are other species that are much more frequent nest raiders.