Triumph Herald

Steed drove the car only once, in series 2 episode Warlock.

It's hard to even identify the car in the episode. It's a two-door car; there were different chassis built.
Steed arrives in the car at Gallion's bookshop.

This photo shows a red Triumph Herald from 1961. And it’s a funny conincidence, because the car appeared in one of the Tintin et Milou books, and John Steed is a great fan of the clever little fellow. He read Tintin et Milou comic books in four The Avengers episodes.

This is a Triumph coupe.
It is one of a light colour, maybe a beige one like this one.