Bulbous Buttercup (Ranunculus bulbosus)
Its not quite that bad as some are very rare now having been affected by intensive spraying of our fields, the Corn Buttercup, once common, is now all but extinct. Other species are quite distinctive in their own way so it really leaves Meadow, Creeping and Bulbous as the most likely choice if it's a standard buttercup you are looking at.
The Bulbous Buttercup is very common on dry grassland, especially on chalk, and so is very common in Dorset.
It does have a distinctive feature that is easily seen. The sepals, found under the petals, turn downwards as shown in the photograph.
So, now all you have to do is tell the difference between Meadow and Creeping Buttercups!