Hi OP: I know it's confusing. I think several of us here have given us the best advice we can, and I've personally been through it with my husband.
Again: A DTP, TdaP, TD, etc will all suffice. You cannot get a single pertussis shot for men in the UK. That is not what the instructions are implying, even if they are confusing. Knightsbridge will give the missing shot required, if you pay for it. As you can see on the visa medical's site. It is £70.
I understand there's confusion about polio. Where polio might not be offered in a combo shot in the states, in the UK it is, as the other poster I linked to mentioned as "Revaxis". That will not be what Knightsbridge will give. For years on this forum there is always confusion about the P for polio vs the P for pertussis, and ultimately it won't matter. What shots I mentioned above will be the shots accepted.
If it were a problem to you and your future spouse, he could easily get a single shot for pertussis in the US from any pharmacy and simply get his vaccines recertified for AOS purposes. I don't recommend that unless there is a general health concern, afterall pertussis is increasingly becoming a problem globally. It is still a moot point, because Knightsbridge can give the missing shot (DTP) needed.
Btw, he will get no assistance from the embassy about this. It's not their wheelhouse. It's Knightsbridge.
Furthermore my husband went to his doctor before his Knightsbridge visit, gave them the list of shots needed and got them. I can't tell you if the DTP or Tdap he received actually was the formulation that Knightsbridge believes it to be, but it was accepted by Knightsbridge and that's all that matters. We also had those same results recertified by a US civil surgeon because of an AOS RFE (that was in error) and it was also just fine. I know it's a confusing, scary, annoying, frustrating process - but do not be concerned about it. Make sure your guy gets the shot at Knightsbridge. Don't leave there without it.