The definition of a 'face covering' in the Coronavirus Regulations is this [quote]: “face covering” means a covering of any type which covers a person’s nose and mouth.
So, as I read it, a face shield alone does NOT meet the requirements of the Regulations.
However the law only requires you to wear a face covering on public transport. Everywhere else it's entirely voluntary (unless, of course, a particular business makes it a condition of entry to their premises that you must wear a face covering).
I've also (theoretically) been 'shielding', due to my ongoing chemotherapy (which, according to the hospital staff places me in the highest risk category there is). However I've been doing some limited supermarket shopping and I'll be doing some more this evening. I've sometimes worn a face mask in Morrison's and sometimes not.
However, as I've never come closer than FIVE metres (not just one or two) from any other shopper or member of staff (other than the checkout assistants, whom I can usually avoid anyway by using the self-service tills), it doesn't really seem to matter that much. That's because I deliberately go shopping during the last hour of trading, when there are never more than half a dozen other shoppers in one of Morrison's largest stores. (There were just 3 of us on my last visit).