The Immigration Rules state that persons seeking entry to the UK are to be refused entry by an Immigration Officer if they fail to produce a valid national passport or other document satisfactorily establishing their identity and nationality (Rules paragraph 320(3)).
This applies equally to applicants requesting entry clearance from an ECO.
A bona fide passport or travel document should:
To enable a UK visa to be placed in a passport, there needs to be at least one full page available which is blank on both sides.