You are biased and want to believe what suits you, it happens with everyone.
The reason why a lot of people cannot believe the £ will rise to the pre-recession levels is because back then, those that got mortgages in Swiss Francs never believed that it would depreciate as much as it has. Lo'n'behold a lot of them are in negative equity with their outstanding mortgage being even 2x the value of their property. They surely never believed/expected that.
Granted, economic cycles pretty much dictate that at some point in the future it is likely that the £ will get strong again. However, intermediate fluctuations that can last for years at a time may see the £ go even lower against the Swiss Franc due to e.g. a British second dip. What happens if you have to sell during those times or find yourself paying a heavy premium on your monthly payment? Actually, given those economic cycles it is very possible that you may find yourself at the next British recession when your mortgage 25 years are about to end, what goes around comes around.
It is a valid (and as history has shown) and possible risk which you need to bear in mind. That does not mean that your idea is not a good one, but recognise that the risk is realistic and don't dismiss it.
Having said all that I think a Swiss Franc mortgage is a rather good idea, especially if you have faith that the British economy will pick up in the long term.
