My guess is that they can't tell their arse from their elbows and understand the difference between professional and consumer supplies in UK law. Consumer purchases give you additional protections as far as fitness for purpose is concerned when you're 'sold' to, professionals are supposed to be competent enough to satisfy themselves as to fitness for purpose and so don't get those protections. What 'consumer' would be looking to calibrate a 7 1/2 digit DVM I really can't imagine (Yes, I know one might be a amateur, but needing a 7 1/2 digit DVM indicates the same kind of competence to make your own purchasing choices as a fully fledged professional).
It should be sufficient to say to them that you can confirm that this is a professional purchase of goods or services as far as the law is concerned. That absolves them of liability. If they are so stupid to not accept that then it's an indication that the rot has well and truly set in, and that they are no longer themselves fit for purpose. As soon as this kind of bureaucratic nonsense sets in it's time to abandon any company that adopts it as a supplier because, while the current example probably won't bite most people in the arse who are buying professionally, who knows what silly rule they are going to make up next that flushes your investment in Keysight gear down the toilet?
I wonder what Keysight think this will do to the resale value of their equipment if people get wind that spares and service are going to be unobtainable if you are arbitrarily labelled as a 'consumer' rather than a professional. Whoops, looks like people did get wind, because I would not be at all surprised if the largest concentration of purchasers of secondhand test gear on the planet are here, reading this.
I'll hazard a guess that a lot of people get to buy Keysight gear professionally because they can sell the beancounters on it having a useful resale value after they've taken the full measure of tax write downs off the taxman. I've had use of this argument myself when getting the company to shell out for a, then, frightening expensive logic analyser from HP. When the beancounters find that Siglent et al have the same specifications, a fraction of the price, and Keysight gear now has no secondary resale value I wonder how much will get bought new 'professionally' in the UK? Hey @EvilDanaherCorporation, what support will your many brands give to amateurs and students buying your kit?
I'm struck by the double stupidity of this. Surely everybody knows that the best way to build a future for a professional brand is to get it into the hands of students. One of the ways to make sure that students won't use your gear is to make sure that there's no support available if you're anything other than a employee of a tidily documented corporation with a purchase order burning a hole in your pocket.