Possible ways to change their minds :
1) Tell them they are opening themselves up to massive personal liability since it is (or should be) well known that both the NHS and Dept. for Education ban them on safety grounds - it's there at the top of the
Fatally Flawed website, didn't realise that both announcements are now around a decade old

If they then plug them in, having been told that they are dangerous, they open themselves up to claims that their insurance company will walk away from.
I'm probably going to live to regret this, but reaching for my Devil's Advocate hat (since I've spent most of my life in 'evidence-based' environments, and feel a bit unhappy if/when it seems that others {particularly 'officialdom'} may be abandoning objectivity in favour of theorising'!) ....
The above is very reasonable,
but only IF the NHS and Dept. for Education (and anyone else who has done likewise) has done adequate properly ('evidence-based') research and sought adequate advice (what some would call 'due diligence' these days) to satisfy themselves that there is fairly strong evidence that these products really
do represent a
net 'danger' - rather than just relying on the rather one-sided and seemingly not-very-objective arguments and material presented by such sources as "Fatally Flawed".
We hear a lot here about 'risk assessments', and I hope these are actually the "
risk-benefit assessments/analyses" that have been a major part of my professional life for decades (on the basis that there are nearly always 'two sides to any story').
As I recall it, last time I looked Fatally Flawed did not actually report (let alone attempt to quantify the frequency of) any actual incidents of 'harm' related to the use of these products, although they certainly did document a number of what they described as 'near misses'. On the other side of the 'risk-benefit' assessment, I don't think they mention, let alone acknowledge (or attempt to quantify, which would be difficult) the possibility that there may well have been some cases in which use of the products has
prevented the occurrence of harm. Without other information/input (which, I admit, would be very hard to find) it's therefore not really possible to be even reasonably sure that the potential downsides actually do outweigh the potential benefits. In fact, at the most basic of levels, I wonder if there have been
any well-documented cases of significant harm resulting from use of the products ?
Then there is the question of the design of the products. I rather doubt that the fact that their pins are slightly larger than those of a BS1363 plug) arose by accident (it would have been just as easy/cheap to make them slightly smaller) - so I personally suspect that it was done deliberately, perhaps because they might at least partially fail to achieve their intended purpose if they were too easy to pull out. Whatever, I wonder if there is actually any evidence (rather than a 'theoretical possibility') that use of these products has ever significantly damaged a socket, or that any such damage has ever resulted in anyone coming to any harm ?
If it were to transpire that the NHS/Dept.of Education/Whoever has 'banned' the use of these products without adequate objective/critical research, such they had actually (due to inadequate thought/research/reason) 'banned' the use of something which actually had the potential to do more good than harm, then those organisations/bodies presumably might themselves be (to paraphrase your statement) "opening themselves up to the possibility of massive liability" ? ... and I would imagine that that 'risk' (of liability) will exist unless they have reasonably strong evidence that the products they are banning are likely to do more harm than good?