Ok, but other than just making assumptions, we have no evidence that the UK did even that here.

My main point is that OP seems to think that trying defend troops already stationed in the region (whether they should be there or not) counts as being part of this attack. Which it really doesn't. As far as I know, American planes didn't even get to take off from any UK bases.

The UK government/military has done plenty of heinous shit over the years, not least Iraq. I'm not pretending otherwise - but on this one, so far, it doesn't seem like they've really done anything.

They haven't condemned it either, of course, but from what we know so far, it's simply factually incorrect to say the UK joined in on this particular attack.

I think defense is part of offense

It certainly can be. But it can also just be defence - if they're sitting there with Iranian missiles heading at them, it's hardly surprising or unreasonable for them to try and intercept and prevent.