Definitely simpliest by car. Guests who arrive by air to Stockholm/Arlanda or Stockholm/Västerås can rent a car. (Sometimes, but not allways, it may be possible to get transportation service from us if we have people avaliable for that).
Please note that there are very inexpensive flights by Ryanair from London/Stansted to Stockholm/Västerås and by Norwegian from London/Gatwick and many other European airports to Stockholm/Arlanda.