Palace considered putting Charles in as Regent as the Queen's health declined

[mamma65432]

https://uk.yahoo.com/style/palace-considered-putting-regent-throne-233044947.html

So it could have really been a possibility and could do again in the future, if William was King before George turns 18 then Harry would be (according to the rules) Regent for George.

That article makes a brief mention of the possibility of a Regency Council. That might be the best solution should there be a need in the next 8 years