Seems totally unnecessary, considering he could just be handed someone else's laptop, right? I'd imagine it would have to be the entire embassy, or else the network that he has access to in his space.
He might work around it but it's an option. I don't know about whether it's legal under UK law but their surveillance organizations seem to get away with a lot.
If GCHQ are involved with any such interference, their actions are fairly unconstrained by law. Paraphrasing Intelligence Services Act 1994, s.(3)[1]:
> [GCHQ shall] monitor or interfere with electromagnetic, acoustic and other emissions and any equipment producing such emissions [...] in the interests of national security, [or] in the interests of the economic well-being of the United Kingdom in relation to the actions or intentions of persons outside the British Islands [or] in support of the prevention or detection of serious crime.
As far as I know, there is nothing in statute that constrains how they go about that (Regulation of Investigatory Powers Act 2000 adds a couple of hurdles in regards to interception). Any oversight comes from various Parliamentary committees.
[1] http://www.legislation.gov.uk/ukpga/1994/13/crossheading/gch...