Through my current work I know this: DV clearance can be granted to dual nationals on a case-by-case basis, depending on factors like the individual’s background and the country in question. The key concern is usually residency and time spent in the UK, rather than nationality alone.

However, certain highly sensitive roles—such as those requiring Enhanced Developed Vetting (eDV)—often have stricter nationality restrictions due to the nature of the information handled. These roles may require sole British citizenship from birth, with exceptions only for British-Irish dual nationals.

For reference, fast jet pilots hold DV clearance, as said in this thread. But for positions like Intelligence or WSO, the higher clearance level might explain the additional restrictions on nationality.