That is why we test, so we know what it is.The symptoms described seem pretty mild though.
Should we stay off work from now to eternity if we have a headache or runny nose?
Coming from what (I think) is a similar political standpoint to @yorkie:
It's sensible staying off work with a fever or bad cough; it's fine to voluntarily stay off work with minor cold symptoms (perhaps because you believe, based on past experience, that the effort of work will exacerbate the symptoms and make you feel worse); but guilt-tripping people into not going into work with a headache or runny nose is quite another. We can't live in this guilt-ridden way from now to eternity; it's time to move on. And given that the UK has little or no in the way of actual legal restrictions, I'd say the feelings of guilt encouraged by some if you have minor cold-like symptoms is the number one problem we need to deal with.
I had what I think was a mild sinus infection at the back end of May. The symptoms (tiredness, blocked nose, "pressure" headaches which were worse when you leant down) were classic sinus so I suspect it was much more likely to have been that than Covid. I did also get pretty bad sinusitis in May 2009 (following a cold) so presumably I am somewhat pre-disposed to it.
(As it happens I was working from home anyway that week so the question did not arise...)