I don't know the price in GB, but the US price for a MaxProp feathering prop is about the same as the price you quoted for the Autoprop at current exchange rates.
My understanding is that an older SM can't fit an Autoprop (they were delivered with a MaxProp) because the shaft is so short the blades can hit the c-drive housing as they flop around.
The Autoprop is certainly in theory (and in their advertising) more efficient, but there is no real-world difference I have ever heard of in boat speed and engine RPM between SM's with an Autoprop and a properly pitched MaxProp at cruising speed.
Also in theory, the Autoprop is more efficient when motorsailing. I can't speak to that, I haven't ever motorsailed for any distance, if the sails are working, I am sailing. If you do a lot of motorsailing, it might be important.
Anecdotally, the Autoprop is more sensitive to fouling which unbalances the blades and prevents them from taking the right pitch. All props are MUCH less efficient with even very light fouling, but the Autoprop seems to load up the engine significantly rather than just slowing the boat down. Again, that is just a summary of my take on the flavor of posts here.
If I had an Autoprop I'd certainly keep it. If I needed a new prop, I'd likely stick with the kind of Maxprop that come with my SM (even if an Autoprop could fit!) It is simple (no bearings), grease can be renewed underwater, and I have many years of experience with it--for what that's worth.
But if an Autoprop was significantly cheaper, I'd go that way.
Fort Lauderdale, FL