Can a bridleway become a general purpose highway?

My client is seeking to buy some land. The only means of vehicular access to it is from the public highway and then across an unregistered area of land (owner unknown) which is shown on the definitive map as public highway. There is no current vehicular access to the land by the route proposed. A new access will need to be created, including the laying of an upgraded metalled surface. The council proposes to carry out works on the bridleway either under section 99 or 278 of the Highways Act 1980 and then adopt the same under the procedure in section 228. However I wonder what powers (if any) the council has to "upgrade" the classification of highway from bridleway to a highway that can be used by the public for vehicular access to our site. The council does not own the bridleway. Can you help please as it seems section 228 only deals with adoption of a private highway/street and not an upgrade to its status.