No, in the UK everyone can tow a 5th wheel (up to 7.5t all up, which covers most Euro-sized rigs) so long as their license was obtained before 1997. The license is more about weight and size rather than hitch mechanism which makes sense, although towing anything big and heavy on a tow-ball is less safe than using a 5th wheel hitch, for obvious reasons.
After that date you have to take an additional test to get the codes on the license, but no need for HGV as such. HGV does however include the lesser license codes, should you already be HGV licensed.
The UK is very keen on over-regulation in all walks of life. I see no reason why anyone with say 10 years of driving experience shouldn't be allowed to tow a 5th wheel! In the UK the overwhelming accident problem is all about young drivers - the elephant in the room that gets ignored. There are very few serious accidents with 5th wheels, even pro-rata - these rigs force you to be sensible! The insurance costs show how low-risk the rigs/drivers are.