UK Sales of Goods Act is not applicable here if you bought it as a download from Smith Micro or a Disc from Smith Micro. only applies to goods purchased in UK.
software also comes under its own rules, downloads have no rules applied to them. packaged software, i believe, relies on whether the data is corrupted ( bad disc).
and because of the millions of different hardware configs, you cannot say  that a program is 'unfit for use' if it runs on most configurations, and just won't work that well on a particular system.
sorry you are having problems.