Depart:
Gothenburg to Marseille with ryanair:
Thu, 6 Aug 09
20:00 Depart 22:40 Arrive
599 SEK/1786,98SEK
173.81€ two adults/ 86,905€ 1
You could possibly take that, but it's really late, dunno if you could make it to the train. :/
Return trip:
Easyjet Lyon to London Gatwick:
Mon 10, August
€72.49
This is how your fare is calculated:
easyJet flight: €58.99
Airport operator charges and government taxes: €13.50
dep. 21:25, arr. 22:00
Easyjet London Gatwick to Gothenburg:
Tue 11, august
£73.99 (87,129€)
This is how your fare is calculated:
easyJet flight: £56.99
Airport operator charges and government taxes: £17.00
dep. 08:00, arr. 10:55
You'd most likely have to spend the night at the airport unless you get housing, you can also fly earlier to London Standsted on same day, but if you don't get housed staying on the same airport would be the best option imo.
Mon 10, August
€79.49
This is how your fare is calculated:
easyJet flight: €65.99
Airport operator charges and government taxes: €13.50
dep. 18:35, arr. 19:15
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87+35+73+88=283€ per person. Add in some dumb ryanair costs, 5-20€ or so, tourney costs, food, traveling via buss/thingies inside sweden and france and all... I think you guys could manage whole trip for 350€ or even below per person (dunno about any easyjet fares). Booking ryanair online with visa electron would make it free, otherwise 5€ each, I think. You could ask fuzzyness to help with easyjet booking.
Other possible routes include flying from gothenburg to london standsted, then easyjet from there or london gatwick to lyon. The timetables however are not so flexible and most likely more costly in the long run as well, unless you are willing leave wednesday 5th evening to standsted already and then easyjetting to lyon from gatwick on thursday 6th (I heard moving inside in london is costly).
Sleeping on the airport won't kill a person, however it would be best if you guys indeed got some rest before actual tourney, so better get housing and train routes secured before booking flights.