As far as next year weather in Europe, anybody's guess would be as good as yours; yours probably better because you have been in the region in May. Go for A/C and don't sweat it (no pun intended).

It sounds like you already know the answer to your question - get AC to be on the safe side.

One other way to look at it might be this: would you ever consider renting a hotel room in Winter that isn't heated?
So, why would you rent a hotel room in warmer months that isn't air conditioned?
I realize that it sometimes can be a mtter of price differences and then there's that elusive "charm factor"..the place you really want to stay in isn't air conditioned, and after all it is May and would you really need it anyway? So in this case you could probably take a chance and be happy.
But I continue to feel that NO hotel is "charming" if you're in bed at night and burning up from the heat (tempreature-wise that is) or just would like a little air circulation.

I'm not sure any of Oxford's hotels have a/c. And it's pretty rare throughout Scotland and Ireland: there are few places on the tourist route in either that aren't close to the sea.

In over half a century of living here, I can't remember it ever being hot enough outside London to warrant a/c in early May.

And I certainly wouldn't ruin my holiday by organising it round hotels that have a/c but are 50 miles from where I wanted to be.

As with everything: over-protect yourself from remote possibilities and you cut yourself off from what you've come to see.

If the outside chance of a spot of heat is too great a risk for you, go somewhere else. Florida, I understand, is good for a/c.

I can't even imagine ever needing AC in London in May, let alone Scotland. July and August, maybe (and obviously yes, this year), but this is a question of odds, I suppose. Why plan your vacation around just an extremely rare event or probability? If you find a hotel with AC you like fine in Scotland, sure, why not book it--but it probably depends what kind of hotel you want to stay at, to begin with, as to how easy that will be to find. I don't consider May to be a warmer month, myself, except maybe in Arizona or places like that.