The Gatwick Airport website lists the following airlines flying on that route:
Viking. (No use to you as the only fly on Sunday)
Monarch. (Again no use. Sunday & Wednesday only)
Thomson. (Useless. Wed, Sat & Sun only)
Thomas Cook (Still no help. Tue & Sat only)
BA: The first actual possibility. Fly out at 0910 (arriving at 1540) for £207 per person - but with only 3 tickets left at that price. Fly back at 1725 (arriving at 2010) for £198. Total fare, per person = £405
That only leaves one airline, which seems to be by far the best solution. easyJet fly out at 0725 (arriving at 1355) for £88.99 per person. They fly back at 1440 (arriving at 1720) for £116.99. Total fare, per person = £205.98
The above fares include all taxes and charges but exclude baggage fees. As with all budget fares, they will rise as seats start to fill up, so I suggest that your daughter should book with easyJet a.s.a.p
Chris