The Foreign Secretary does have statutory powers under the Diplomatic and Consular Premises Act, 1987 to remove the diplomatic or consular status of any premises. As s. 1(3) provides:
In no case is land to be regarded as a State’s diplomatic or consular premises for the purposes of any enactment or rule of law unless it has been so accepted or the Secretary of State has given that State consent under this section in relation to it; and if—
(a) a State ceases to use land for the purposes of its mission or exclusively for the purposes of a consular post; or
(b) the Secretary of State withdraws his acceptance or consent in relation to land,
it thereupon ceases to be diplomatic or consular premises for the purposes of all enactments and rules of law.But as both Roger O'Keefe and Carl Gardner have explained, these powers are circumscribed by classic principles of administrative law: they can exercised only to further the purposes for which they are provided.
Section 1(3)(a) is essentially a cleaning-up provision, to be used when a diplomatic or consular mission changes from one premises to another. As for s. 1(3)(b), s. 1(4) further provides that it may be exercised only where the Foreign Secretary is satisfied that it is permissible under international law to withdraw acceptance. There is no credible international law case that withdrawal would be permissible in the current circumstances.
Getting to the airport, or into a helicopter and out of British airspace is quite another question, however. I hope Mr. Assange likes canned goods...