I think it is worth reading the whole statement from MI5 and GCHQ.
Sir Ken McCallum, the MI5 director general, and Anne Keast-Butler, the GCHQ director, say:
MI5 has over 100 years of experience managing national security risks associated with foreign diplomatic premises in London.
For the Royal Mint Court site, as with any foreign embassy on UK soil, it is not realistic to expect to be able wholly to eliminate each and every potential risk. (And even if this were a practicable goal, it would be irrational to drive ‘embassy-generated risk’ down to zero when numerous other threat vectors are so central to the national security risks we face in the present era.)
However, the collective work across UK intelligence agencies and HMG departments to formulate a package of national security mitigations for the site has been, in our view, expert, professional and proportionate.
Referring to the steps that have been taken to reduce the risks, McCallum and Keast-Butler say:
As detailed in classified briefings given to the intelligence and security committee of parliament, the package of mitigations deals acceptably with a wide range of sensitive national security issues, including cabling.
These mitigations will be subject to regular review through a cross-government process, led at senior level in the Home Office.
Further, it is worth reiterating the new embassy will replace seven different diplomatically-accredited sites across London which China currently operates; this consolidation should bring clear security advantages.