Let's say the company's headcount to develop/maintain such software, plus a bunch of other required roles such as marketing, sales, hr, etc., is around 25 people averaging £80,000/year each (not that far off considering employees cost more than just their salary) to make it an even £2 million/year.

That means in order to not go bankrupt, i.e. break even, they need to sell 10,000 copies to new buyers every single year. Is that realistic? I don't think so, a company that tries that model will soon go in the red because we aren't even considering other costs that a company has, which aren't negligible. Even if you cut the estimate above in half, e.g., let's say they cost £40,000 each instead, that's still 5,000 new buyers of the software each year.

In contrast, a subscription model of £100/year requires them to have 20,000 ongoing subscriptions, which is substantially easier to achieve. I believe there's a reason that a lot of companies have shifted/are shifting to a subscription model, an alternative being monetisation models such as advertisement-supported programs.