I'm not sure in law it is a sexual assault.
The offence is
Causing a person to engage in sexual activity without consent
(1)A person (A) commits an offence if—
(a)he intentionally causes another person (B) to engage in an activity,
(b)the activity is sexual,
(c)B does not consent to engaging in the activity, and
(d)A does not reasonably believe that B consents
Presumably this was a thank you not sexually motivated. I also think it's not beyond the possibility that a woman would do the same to a man in similar circumstances although I expect posters won't agree that that would ever happen.
It's utterly misguided in the present circumstances - they could have just gone for assault.
A kiss on the cheek in the pandemic is herrendous. whether it would qualify as a crime the same way than coughing at people is, I am not sure
I don't see why not- it's worse if anything as it's direct contact.