I already have a long term visit visa but my passport is full. There is space on some pages for an immigration stamp but no entirely blank page.
I am looking for authentic answers from official sources. There isn't enough time for a passport renewal.
Well, official sources aren't normally going to explicitly say when a certain requirement doesn't exist.
TIMATIC, the database used by airlines, points out if there is such a requirement for a country (as airlines get fined for transporting inadmissible passengers), but doesn't mention this for the UK.
Therefore, as long as there's space for the entry stamp, you should be fine.
