Yeah I don't like driving on space savers for a long way, I would avoid doing length of the country on a space saver (you then have no spare right? If the spare is v old it may not last that long at 60 mph, rubber does die with age not just use). They can only repair punctures in the tread, if the sidewall is damaged they won't/can't do it.
I second the just get another full size wheel option if you can't get your tyre repaired, but if the bolt pattern and offset is right (the spare wheel physically fits, plus it clears the brakes etc.) using a washer will work in a pinch if you really are stuck.. Make sure its at least grade 8 though, don't use cheap alloy shit, and I'd put some loctite threadlocker stuff on it so it doesn't vibrate loose, you don't want it coming loose on the motorway. I dunno if I'd drive across country like this though (unless I had to and waves were pumping haha, and then I would check the tightness periodically..). With a washer on there though you might find that the wheel is no longer perfectly centred, you'll want to make sure you tighten it properly in sequence, if it isn't centred you may get some vibrations at speed.
I have done some pretty sketchy shit to cars, we broke a suspension mount on my mates 4 wheel drive when we were in the middle of nowhere, had 2 vehicles there so we used the car batteries hooked up together to weld the mount back on. It worked, and we just welded it properly when we got back to town
