All UK practice of course - different in the USA.
And in most of the rest of Europe which, unlike the UK, complies with EU law.
Does that mean an a/c with just one seat ?
No, according to FCL.010 it means an aircraft certificated for operation by one pilot.
How do we know that a 'single pilot' aircraft is a single pilot a/c ?
You look at the
EASA Type Ratings and Licence Endorsement List