This 5% chance is too low. The other nations are much closer to leaving, and as long as they dominated by that, and without a collapse in the Tory vote in richer parts of England, Westminster will be controlled by a centralising patronising Tory cream. There will be little incentive to remain in the UK, and as a nation, it is easier to imagine Wales leaving than Lancashire.
@CromlynGames There's 7 years for this question to resolve YES. That may seem like a long time but there is very little serious agitation for Wales to leave the UK right now, and these sorts of movements take years to build up steam. The scenario you suggest is broadly plausible by 2040, or maybe even 2035, but even if Scotland leaves a couple years from now, I seriously doubt pro-independence sentiment in Wales could rise fast enough to cause an exit by 2030.
Addendum: Given that the SNP so throroughly dominates Scotland electorally, their leaving would not actually significantly alter the pro/anti-Tory balance of power in the remainder of the UK.