Nice idea but groups are not unique unfortunately.
King John and Henry VIII could be swapped.
Also another possible link for John, James II, William IV, George VI (and Henry VIII) is that they are the last Kings with those names.

I don’t get this ‘non unique’ answer business.
Isn’t that the same as crossovers? Surely a good grid has lots of these!
Of course that makes the grid harder, which some people don’t like.

All grids should have a unique solution Jahn - so there is only one way they can be put together for all the links to be accurate. probably the most common flaw for us bumbling amateurs is to feck up this crucial requirement.
Red herrings are what make it a puzzle beyond just simple knowledge. But let's say there are the names of 5 Kings on the grid - 4 fall into the Kings of England link; the other into "They're all names of vacuum cleaners." That's great. But if one of the kings that you have in your King group is ALSO the name of a vac, ie. so there are 2 vacs with kingly names, then you have a faulty grid. Those two names can be swapped around. You have a non-unique solution.

The example Jonboy gives though isn't a good one. Henry VIII and John aren't swappable. But the chances of a unique grid with 16 things of the same type are slim.

Thank you! That is a very helpful explanation BinDipper.
I was very confused by the wrong example from Jonboy of Henry VIII and John (maybe he was thinking of Prince Arthur perhaps?).
But yes, I can see there are lots of monarchs who succeeded their fathers which spoils the picture.
Will try harder next time!😀

Are you referring to king Dyson of Lichtenstein 1492-1588? Or Dyson King of Prussia? The former succeeded both his father and his brother as they were the same person.

I also found this:
Harald "Upright" Gormsson (Old Norse: Haraldr Gormsson; Danish: Harald Uåpstand Gormsen, died c. 985/86) was a king of Denmark and Norway. He was famed for standing in a corner until his enemies got embarrassed and left. The upright vacuum cleaner is named in his honour.
I hope this is helpful ;)

Only the one English one though!

Well, eight?