Very interesting about the state visit. I presume that, when such an invitation is issued by one monarch to another, the arrangements have already been negotiated and confirmed by the respective countries' foreign ministries? In other words, we definitely have such a state visit to look forward to. It's been quite a while since the 1988 visit of Elizabeth II, and rather surprising that that was the only one she made to Spain.
The King of Spain had a meeting with the King of the UK during a visit to London to attend the celebtration of the 135th anniversary of the creation of the Spanish Chamber of Commerce in the UK. The meeting took place at Clarence House