Hi Raj,
That road is almost certainly 30 or 40 mph. Any 40mph zones will be clearly marked with 40mph boards as you enter them and along the route.
My wife and I often visit that area and the speed limits are almost irrelevant. It's hard to find anywhere else in the UK with as much traffic as Purbeck and at all times of the day. There even seems to be more traffic than in the southeast. The A351 is the only major road through Purbeck and if you get above a crawl you will be doing well. It's even worse during busy times such as when the schools come out!