While you can certainly get mild at some pubs, I think it's only fair to warn you that the majority of pubs don't sell it.

Some folk clubs close in August but they are all back in September.
If you're going to be in Suffolk, get hold of a copy of Mardles magazine which will carry listings for folk clubs and sessions in Suffolk, Norfolk, Cambridgeshire and Essex. The aforementioned Blaxhall Ship Inn is one place where it's on sale, but contact me if you need help in tracking it down. For Hertfordshire, try Unicorn Magazine.