Affordable restaurants on the Strand

The Delaunay on Aldwych, about three or four minutes walk away, would be my choice, especially if you’re looking for a pre-theatre meal.

https://www.thedelaunay.com/menus/

However you have all of Covent Garden and its multiple eating places close by.

It very much depends on what you mean by “affordable” and the kind of food you want to eat.

What is “affordable” to you? £10 a head? £50? £200?

There's Ivy Market Grill at Covent Garden which is nice. Also a good Italian Place called Prima in Tavistock St. But if you wanted somewhere cheaper the there are plenty of burger places and pubs as well.