The Land Registry entry replaces the deeds but all the relevant information from the deeds should be incorporated in them. The Land Registry only indicates what they call general boundaries. These are not precise but serve for most purposes. If the exact boundary needs to be determined then the services of a specialist surveyor are needed. This site is a good place to start if you need to.
There are no hard and fast rules and what may seem obvious from the plan may not be correct. As an example when I purchased my current house my solicitor queried the ownership of the small pull in that is in front of the gates. The local highway authority claimed ownership although the Land registry plan suggested otherwise. This was in a poor state of repair and although the road had recently been resurfaced the pull in was seriously potholed. In fact it was one big pothole! So I asked the them to repair it. The reply was it was not their responsibility. The letters claiming ownership and denying it were signed by the same person! After further correspondence the repairs were carried out. Persistence counts.