I suspect that they will be mongrels of some sort - it seems unlikely that they will have not been in contact with bees in other parts around the Blenheim estate - there has to be some overlap. Having said that bees are not kept anywhere on the estate so these bees are going to be feral of some sort.
I suspect that they may be low in varroa - small colonies, swarming regularly and foraging locally - the likelihood is that they will not be excessively exposed to varroa and the resultant vectored diseases. The multiplicity of natural hives in this ancient forest is clearly nearer to the natural environment that bees had access to in times past.
Personally, I'm not sceptical that these bees exist and thrive... I'm sceptical that they are 'survivor' bees from pre-1918 but I do think, in this location, they could have avoided (at least in part) being affected by varroa ... it's a pity he didn't do a few sugar rolls to establish if they are devoid of varroa rather than concentrating on the 'survivor bee' angle.
Bees are opportunists - they won't fly miles if there is good forage on their doorstep and the area around Blenheim has some excellent forage - if they are not flying far enough to interact with other bees ... who knows - they may be ferals that are surviving ?
It's a good story - I'd rather take the positives out of it rather than disparaging it.