The real theme

I thought it was about a car race.

Perhaps two of the last moments in the film may illuminate the point?

When the courteous old gentlemen starts enthusing over Genevieve, first Alan and then Wendy fall under the spell of his love for the old car and of all that it meant in his life, in particular his relationship with his wife. So touched are they by his depth of feeling that they halt the silly race and give him some time instead. Jerked out of their post-war aggressive self-absorption, they respond sympathetically to someone from an earlier age with more civilised manners.

Then the other pair overtake them but, once their wheels get caught in the tramlines, they are turned 180 degrees towards Brighton. Brash promiscuous adman Ambrose epitomises the new era, just like vacuous and probably alcoholic model Rosalind. Symbolically, they do not reach the finishing line but instead are pointed back to the town that was the nearest England had to Sodom.