Oddly enough, the expert adviser's column you linked to, pretty much parrots what I tell folks when these types of products are marketed, here in the states. We generally don't have a lot of homes that feature painted exteriors, but plenty of shady outfits offer "magical" roofs, siding and windows. Typically at 2-3X the value of an honest installation, by a reputable local contractor. Invariably, when issues with the products occur, years after the salesman got his commission, customers end up unhappy. The warranty rarely is honored for a variety of reasons, from the fact that the contractor no longer exists, to some weasel language in the original warranty that allows the contractor to squeak out of standing behind the product.
As you might of guessed by now, I'm not a fan of the idea. It sounds like the combination of older masonry buildings and the lovely UK climate results in exterior coatings that last last 4-5 years. I would just budget that amount in coming years, and see if any of your neighbors have had success with paying double for a "magic coating" Good luck.