May have missed the obvious
If those UK passports pre-date the issuing of biometric passports (IE no chip) then we are looking at plain passport fraud.
However if *any* of them were chipped this would imply either a failure to pick up 2 requests for a passport (the genuine and the impostors) or (the passport is a forgery) and somehow someone has faked the authentication code held in the chip for the persons photograph.
The former is a failure of the current issuing procedure (which *should* be fairly easy to fix) the latter is a serious compromise.
Note faking the authentication is AFAIK pretty easy *if* you have a copy of the 2 lines of machine readable text in the back of the passport. Which if these people have gone through Israeli passport control would be fairly easy to copy.
It seems the root cause flaw is the *assumption* of trust of even a *permitted* passport authority. Even people who have a legitimate reason to access those lines *might* misuse that information., by deriving the access code for the JPG to modify it.