In UK law it is termed "used for human habitation" so parking? or camping? are the same thing. The person in the van is not subject to legal penalty for parking on the land. The UK has not recognised the EU ruling other countries have, such as Spain. EU laws have to be ratified by each member country to have it incorporated into its laws.
John, I don't think that's quite factually correct. EU Legislation overrides national law. EU Legislation comes in two different forms, regulations and directives. Regulations become law in all member states the moment they come into force, without the requirement for any implementing measures, and automatically override any conflicting domestic provisions. Directives require member states to achieve a certain result while leaving them discretion as to how to achieve that result. The details of how they are to be implemented are left to member states.
With regards to Wild Camping it is difficult. Alan states a motorhome is a car. This is I believe based upon the EU definition of a motorhome under category M1 as, in brief, a vehicle with less than 8 seats adapted to include living accommodation (see European Union Whole Vehicle Type Approval), but this is not law. It is not a regulation or a directive.
I think therefore the UK term Motor Caravan (a motorhome does not exist in UK law) is the relevant term here. This makes it subject to the Caravan Sites and Control of Development Act 1960, which as far as I can see has not been overridden or amended. As John correctly states, and as implied under the EUWVTA, the "human habitation" also plays a part, but the legislation is from 1960. If you want to read the full text, it is here:
Caravan Sites and Control of Development Act 1960
There are a very few exemptions for "stopovers," but basically they are not allowed.
However, as always, if you use common sense and don't make a nuisance, then the law is likely to accommodate you much of the time.