I'd say if you intend to keep the car tucked up, all clean and shiney, under a cover in the garage and just bring it out every other sunny sunday, then I'd say yes. It's definately worth it.
If you use your car daily in all weathers, you will probably be cleaning it weekly anyway, so it won't matter so much if it gets dirty. If that is the case, I would see the £50 as a tank of fuel every month.