Good option for protecting Brooks saddle
Brooks leather saddles should not be ridden in heavy rain without some form of protection from getting drenched - otherwise the saddle will get stretched out and ruined. I have used the Dry Ride covers for years to protect my saddles; while on tour, they work well to cover the seat when camping and the bike is left outside all night in the rain. Some people prefer to use a cheap plastic bag, but I prefer the Dry Ride because it is durable, elasticated in the right way, and it "just works". It doesn't fit absolutely tightly on your saddle, but in practice the loose parts flatten down and you don't notice it's there. The extra space allows it to fit lots of different saddle shapes, which is useful. Recommended, I always carry one of these in my handlebar bag, ready at hand for heavy rain.
Note that this will not protect the underside of the saddle from road spray from your wheels; however a set of fenders will take care of that.