The only operator from Luxembourg to Gatwick (which is the closest major airport to Hove) is easyJet. Unfortunately they don't operate on Tuesdays and Wednesdays, this evening's flight is sold out Thursday's flight is also an evening one so, unfortunately, that's the most obvious route ruled out.
BA fly from Luxembourg to Heathrow but their first flight on Thursday doesn't arrive until 1145, so it would be necessary to travel on Wednesday. That offers the option of departing from Luxembourg at 1115 (arr LHR 1145) for €252 or at 1805 (arr LHR 1905) for €309.
Luxair operate from Luxembourg to London City. There are flights at 0645 (arr 0705), 0715 (arr 0730) and 1050(arr 1105) this Thursday but they're all priced at €399.79.
Ryanair operate evening flights from Luxembourg to Stansted, so that would mean travelling on Wednesday on the 2140 flight (arr 2210). The fare is far better though, at just €26.51.
There's no train from Stansted to Brighton at that time of night though (unless your daughter is prepared to leave Stansted at 2315, to arrive in Brighton at 0320, after taking a train, a tube service, another train and then a bus), so an overnight stop in London would be needed.
There are no other flights from Luxembourg to the south of England. Looking at other airports in the region, Metz (90km from Luxembourg) doesn't help, as there are no flights to the UK. Frankfurt Hahn (113 km from Luxembourg) is possibility though, if your daughter can get there for an 0645 flight on Thursday morning with Ryanair (for €46.91). That would get her into Stansted at 0710, leaving plenty of time to get to Hove. If she'd prefer the same flight a day earlier, it's priced at €57.11.