Obviously there for a reason, but our kitchen is fricken freezing at the moment, due to two outside vents behind the cupboards. One is 3 inch off the floor and the other about 18" above that.
.
Can I block one the lower one temporarily (during this cold snap) without any ill effect, or even both?

The vents are usually put in there to allow carbon monoxide to vent out and fresh air in. Newer boilers don't need a vent.
What do you have in the kitchen? I'm assuming a gas cooker and probably an old boiler with that much venting?

Gas cooker and a newish boiler.. hmmm

In that case I assume you've got cavity wall insulation too?

Personally I'd block them up and get a carbon monoxide alarm. Might be worth speaking to a plumber who knows what he's doing too.