Fortunately I don't need visa. But you're right that it's a nightmare how to organize this remote purchase + flight.
Regarding Decathlon: while that is something I did consider, but... Currently I am only going for 10 days or less, so I would end up with a bike that is impossible to sell there (especially considering that I'll fly back so I will need to sell it on the spot, so it's only possible in some bike shop. but then I would get maybe half price after 10 days of use). If I don't sell then I'll need to fly it home, but then I'll end up with a bike I could also buy here in the local Decathlon, and I don't really need/want it on the long term.
Another option I considered is to but a second hand city bike (price < GBP300). It's not a bike that I would use for a 2 months tour but for 10 days it's probably OK. And it would be possible to re-sell it in a bike shop (maybe even in the same one I purchased it) with a loss that would be comparable to renting a bike for 10 days.
And I could maybe just rent a bike for 10 days, but then I might end up anywhere in Europe (and maybe in September I would then look further south)
Another option is: to buy a real touring bike (used) that seems to fit my plans for the next few years, and keep it after the trip, then upgrade it if needed before the next year's bigger trip.
Of course the charm of the last option is that I end up with a nice souvenir
but it's also the hardest to plan and execute, and probably also the most expensive, but it's a good investment for the next few years.
Below this is a bit off topic, but I hope the OP doesn't mind
What doesn't help my decision (which touring bike to buy) is there are at least 2 price ranges (based on resent sales in ebay, gumtree, facebook) I might consider:
GBP 350-650 - I saw some Sherpas, Nomads (both mk1) or Club Tours
GBP 700-900 - Ravens
GBP 1000+ newer Nomads, Ravens,...
Given that Thorn bikes are almost always custom built it's also very hard (at least for me) to compare them. I started a Google Docs where I list all the features from the ad + what I can see in the pictures, but it's very hard. What is more important? Dynamo hub + lights or Rohloff gears? Or is Alfine fine?... And then there's also the sizing...
There are some bikes currently being advertised in this forum that look decent, but similar (at least to my eyes) bikes were sold 30% cheaper in the past month on ebay. I don't really know if the bikes in ebay had some drawback that I don't see, or the winners were really lucky or the bikes here are too expensive compared to the market.