I wonder how this consistency, and the figure, comes about. And if it were covered by the national minimum wage legislation whether it would be legal (the same could also be asked of other sorts of academic examining).
In the social sciences examining a PhD almost always involves reading a thesis of between 50k (for doctoral programmes) and 90k words, evaluating it, writing a pre-viva report on it and preparing questions for the viva. For an experienced examiner that takes a minimum of 6 hours, but for others considerably longer.
Then there's the viva itself - travelling to and from the venue, having a meeting with the internal examiner, conducting the viva, discussing the result with the internal examiner, meeting again with the candidate to inform them of the outcome and any requirements. That's usually a whole day - say 8 hours. More if an overnight stay is required.
Then there's the post-viva report to agree with the internal examiner, write and approve. Another hour.
So that's around 15 hours in total for an experienced examiner.
Perhaps that's the source of the consistency in the fee level. £10 per hour (roughly). It passes the minimum wage test (currently £7.20 an hour for those over 25 - which examiners will usually be and experienced examiners certainly will be). But it will almost certainly fail that test in the case of a less experienced examiner who may devote a couple of days to reading and considering the thesis.
If you were picky you might say that travelling to work doesn't count. That may be true for those on wages and salaries, but not to the fee-based consultants. They charge for their time. Naturally - it's a question of opportunity cost.