To Stockholm Arlanda you can fly with either SAS or Norwegian.
There are no direct flights from Manchester to Gothenborg, but both SAS and EasyJet fly to Copenhagen. From there you can get a train (www.sj.se) or bus (www.swebus.se) to Jonkoping (change of train/bus is almost always necessary) - you can also take train/bus from Arlanda..
OK! If we are to use Ryanair+rental car for this it would be better to find flights to Stockholm Skavsta (NYO)
The drive to Jönköpnbg from Skavsta is only 2.15!
More flight to choose from too! And more cars!
Again, i am not a Sweden expert.
But when we arranged a 9 day house exchange UK to Sweden, which we are on now.
Our dates were flexible and Fridays from Stanstead returning on a Sunday was 67.
We are 6 in 2 cars so the flight price was more important than car journey length.
The cheap Stanstead to Vasteras flights were also at sensible times..
Outward 11.40 return about 15.00.
So you got a good deal on that day! Good for you!
When I give advise on TA it must be more universal.
I did a search now and as an example I took august 13.
Stansted to Västerås or Skavsta are exactly the same price!
I´m just presenting the options to travellers who are not familiar with the geography and transport choices in Sweden.