SkyScanner is a good site but it's not infallible. It never shows the really cheap flights when, say, Ryanair has flights at just �0.01 (+ taxes & charges) each way.
I often fly with Ryanair, from Stansted, and usually pay under a tenner (+ t&c) each way. (Most fares I pay are usually in the range �0.01 to �0.99). However, I'm able to book early, fly mid-week and select destinations which aren't the most popular.
Alicante in September is bound to be popular and it's already quite late to find really cheap flights. Even so, by trying a few sample (mid-week) dates in September, I've had no difficulty in finding return flights, from Stansted to Alicante, for a total (including all taxes & charges) of �395.64 for 4 people. Roughly similar fares are also available, with Ryanair, from Liverpool.
However, Jet2.com can do it cheaper from Leeds-Bradford (�352.76).
Even better though, easyJet can fly you from Stansted to Alicante (out 13/09, back 27/09) for a total of �291.92. (easyJet also fly to Alicante from Bristol, East Midlands, Edinburgh, Glasgow, Liverpool, Garwick, Luton & Newcastle).
Chris