SIF at RAF Cosford do some cheap forage caps. They are good for traveling (saves crushing you SD hat in your luggage - I note that Gieves and Hawkes's is only £190). Especially as berets are not an option in blue for mere mortals; unless, one belongs to the Corps (that's RAF Regt not Air Training).
Fashion tip - don't wear it (the forage cap or (as I have seen) the SD hat) with combats - especially in the company of the Army - it just makes you look a complete tw*t. Dig out the beret they issued at Cranwell or hang the expense and by one with a decent badge.