I know you're not asking, but in my experience, valentines day is not the same here as it is in the US. Very much a couples sort of thing, far less (or none) of the general love/like, hallmark-driven holiday it is in the states.
Also, it's a bit more sexy, like everything in the uk.
So your coworkers might not quite get an outpouring of Valentine's Day appreciation.
Or I could be being a Debbie Downer and it's a really cute idea. I dunno.