You need to be aware that there is a legal base

[chandon55]

What about stopping the car? Stopping the car is, arguably, outside the prohibited actions in Art. 22(3). But if the car is stopped, then what? The car cannot be searched. And the members of the mission have freedom of movement (under Art. 26, VCDR). In any event, as the BBC says, even if Assange got to an airport or sea port, he would have to get out of the car at some point.Enter at Your Own Risk
Written by Roger O'Keefe
Roger O’Keefe is Deputy Director, Lauterpacht Centre; University phone number library Senior Lecturer in Law, University of Cambridge and Fellow & College Lecturer in Law, Magdalene College, Cambridge.

Ecuador has alleged that the UK has ‘threatened’ to rely on the Diplomatic and Consular Premises Act 1987 (‘DCPA’) to enter the Ecuadorian embassy to arrest Julian Assange, who has taken refuge there in order to avoid his extradition to Sweden (see, e.g. a Guardian report here). In a letter said by Ecuador to have been delivered through a British embassy official in Quito, the UK government is purported to have stated:

in the UK, the Diplomatic and Consular Premises Act 1987, that would allow us to take actions in order to arrest Mr Assange in the current premises of the embassy.

We need to reiterate that we consider the continued use of the diplomatic premises in this way incompatible with the Vienna convention and unsustainable and we have made clear the serious implications that this has for our diplomatic relations.

We only have Ecuador’s word for it that the UK government has made this ‘threat’, and we should be cautious in accepting this without corroboration. But let us assume for the sake of argument that the allegation is true.