I do agree with these sentiments, (and it doesn't only apply to UK BTW). BUT .............
We (all of us) complain that when we order spare parts we have to wait, sometimes for weeks! So a dealer or distributor whose main business is selling, say, circular saws, is actually investing money "today" for something he may not be able to sell (so that he gets his money back) for months, or maybe even years! In other words, there is a definite cost in holding spares on the shelf for an indefinite period.
BUT (again!) we all know that the cost of borrowing capital (interest rate) is much lower these days than it was in the past. AND we shouldn't forget that the dealer or distributor who does invest in such spares SHOULD be able to recoup at least some of that extra cost because he'll increase his sales overall because of good customer service reputation.
So it comes down to "how much does it REALLY cost to make such spares investments?" That depends on a lot of factors, including the original cost of the spares, the mark up (profit plus), the amount of space taken on the shelf, and the frequency of turnover. But that last takes a lot of knowledge and experience to adequately "guesstimate".
So what's fair really? I dunno, it depends on all the above factors, but the mark ups mentioned do seem, at first sight, rather high to me (but that's without knowing all the facts as above).
One thing I saw a lot before I finally retired from the aircraft business (where spares really do cost an arm and a leg!) was that a lot of "genius newby consultants and bean counters" seized on the idea of "let's reduce spares holdings, it'll reduce our costs" (like it was a really clever, original idea)! 99 times out of 100 it wasn't of course, it just increased flight delays, which cost a LOT more in the long rune than holding spares! Experienced "guesstimating" again.
Anyway, just to say that there ARE costs in holding spares on the shelf, but as said, there ARE often savings to be made when it comes to buying "standard" spares like, say, switches, bearings, etc, from a specialist distributor rather than the tool manufacturer or dealer (normally NOT an option in the aeroplane business BTW).