This is a tough question because no English monarch after Henry VIII tried to behead their spouse, and it's an issue where until the monarch actually tries to do it, there's no way to know if they would be successful. Bear in mind that Henry VIII had to actually lay charges against both wives, he couldn't just decree that their heads would be taken off without some legal excuse.

So after Henry VIII you've got a period of short, unstable reigns, then Elizabeth, then the Stuarts. I can't think of an instance where a Stuart king would have much basis to seek his wife's beheading. Then I suspect a definitive cutoff would be the transition from James II to William and Anne, as 1) They were joint monarchs and 2) during their installation a fair amount of unilateral monarch authority was taken from the crown.

Considering what thecarebearcares says about Henry VIII having to lay charges that were capital crimes against the wives he had beheaded, theoretically it would still be possible for an English monarch to "have their spouse beheaded" (well, executed?) up until the death penalty was abolished in the UK in 1998.

I mean presumably if Queen Elizabeth had solid evidence that Prince Phillip was guilty of a capital crime (espionage, treason, mutiny, etc, also including piracy, lol!), she could have "had" him executed, Henry VIII style, up until 1998.

The real difference in the case of the reign of Henry VIII is that he had the power to convince the rest of the government to find two of his wives guilty of capital crimes (of which there were a lot more in the Tudor era, as well). Note for instance that he didn't at all try to get rid of Catherine of Aragon this way, because he just didn't have the power to do something like that.