They may be Saudi schools, they are on British soil.
No, they are not. The school(s) may be inside the UK, but Saudi Arabia bought the land, and almost certainly has a GREEN ('religious') Islamic flag there on display, even if only indoors.
Under Islamic law, that makes it a part of Dar al Islam, ('the House of Islam') though not of Saudi per se; but Islamic soil none the less.
Now and forever, it 'belongs' to the Umma (world-wide body of Islamic believers), and is subject to rule by the Uleema (the Islamic religious authorities).
They MAY dissemble, and 'allow' the Brits to exert control over it for now; but, if, or when, they get strong enough, then they will make that doctrine perfectly clear to any infidel (take your pick: ape, pig, dog) less well armed than they are.
In their eyes, they have stolen that territory fair & square, in the name of their moon god, better known to us as Satan.