Thanks - I was hoping to negotiate 3 days there, 2 days working from the UK. Not sure if this will be do-able but figure it's worth a try.
My plan was to try a few air bnbs, find one I liked, and then make a standing reservation with the owner. Once costed out, it actually works out about the same price to do this as to find an apartment in Stockholm (if that's even possible!) and live there full time with visits back home every few weeks.
If I plan to do this, would people recommend trying to achieve silver asap? Does the lounge make a difference or do you end up getting to the airport so close to departure anyway that you don't really use it?
Definitely go for silver (won't take long to get those 50 flights if you go weekly). If you live in Oxford you need to add a margin for traffic delays in getting to the airport, so you'll always arrive early enough to have breakfast in the lounge (and at LHR it isn't bad - porridge in winter etc).
It's a pain commuting from the UK to the rest of Europe rather than vice versa because of the time difference which acts against you. I commuted to Paris (on the Eurostar but the overall timings are similar) for three years and once you get used to the early Monday morning start you'll be fine. Key is to agree at least one day a week working in the UK. If you can agree two, then that's only two nights away which is even better.
Never ever agree to fly out Sunday evening - that wrecks your weekend. Instead, accept that on the two (or three) nights you are in Stockholm you'll be working late into the evening to compensate for the late arrival in the office.
Make sure you can arrange accommodation where you can keep clothes etc while you're not there. Then you don't need to travel with anything more than you would go to work with in the UK. Airbnb will include cleaning services and most cleaners will also iron your shirts for you if you pay them a bit more - that saves a lot of hassle as well.
Good luck!