They will forward a letter if you provide a stamped envelope to do it.
I've had excellent responses through this agency
https://thebwhagency.co.uk/contact/

I wrote to him many years ago about Smileys' People and got a lovely letter in reply.
A real gentleman.

I have never manager to get a Fan mail response from IAG.
I have tried twice for Cassie through them following her appearance in "Endeavour" but no joy!

Alan is normally really great at signing via venue.