It's applicable, but it's not a real world example because nobody has the same mortgage rates and interest rates over that period. Over the last 25 years the BOE rates have gone from 6% in 1999, all the way down to 0.5% in 2009, and back up to 5%-odd whatever they are now. How can you do the sums on what you don't know for the next 24 years of your mortgage?
Besides, if you want to calculate it over the whole period you need to take into account your max overpayment amount as previously mentioned which is generally 10% of your loan. So you have to "reset" your sums every year as you take out that amount to overpay. Some people won't actually be saving more than they could feasibly overpay anyway, in which case you may as well be doing your sums year on year basis. In which case as above if you're talking about £100-200 difference per year it might not be worth the hassle.
But then like I say above it's doing your max overpayment every year, changing savings accounts every year to maximise the rates available, moving that money. Then doing these sums every time your rates change on the mortgage or savings. Yes it's not that much admin in the grand scheme of things but it's tedious nonetheless.
Each to their own. We're only going to go round in circles on this. We've done the sums, double, triple checked them separately and are pretty comfortable that the difference gained is not worth the aggro in our particular circumstances. We spent years and years and years admin'ing savings accounts, moving around for the best rates etc. It's a pleasure to not have to do that for once, and simply start attacking this big 'ol mortgage we have instead and watch it go down.