'Ladies Of London' Star Juliet Angus's Bday Bash

"Ladies of London" star Juliet Angus is turning the big 4-0 in a $15k a night, members-only English countryside estate.

We're told the party's going down the weekend after Thanksgiving at hoity-toity 10 Castle Street, about 2 hours outside of London ... not far from where Guy Ritchie tied the knot last year.

Guests will be treated to a Thanksgiving feast and bestie Lindsay Lohan -- if she shows -- is sure to hit the decks for the party.

There's a strict dress code -- velvet only. Activities include horseback riding and croquet. There'll also be massages upon arrival.

Only her favorite "Ladies of London" costars are invited, which might provide clues on season 3.