Back to my planning---

So 6ish sounds better. A drink while deciding on our menu choices, dinner & dessert, should get us out in time.
So far, Hawksmoor looks good

Any other suggestions?

Alternatively, you could splurge on a good lunch and just snack before and/or after the performance.

Or - an afternoon tea at one of the posh hotels. The full tea would be tea (w/ or w/o champagne), sandwiches (essentially as many as you want) scones, cakes, etc. it really is a full meal. Booking a tea at 5 or 5:30 (which is normal while dinner isn't really) would be about right. A full afternoon tea takes about 90 minutes.

No one will be hungry. Then if any of you are a bit peckish after the show, they can stop for an after theatre supper . . . Or not.

Not sure of proximity to your theater but what about Cote? We enjoyed dinner there when seeing Oliver back a few years ago.

When we did a late afternoon tea at Claridges we were so full that that was our dinner.

We saw Matilda in New York with the two folks (the man who played Mrs. Trumble and the lady who played Miss Honey) who had won the Laurence Olivier Award originating the roles in London--they came over for a short while during and right after previews. The show is wonderful. I think you will enjoy it very much.