A and C
Landing on a runway which is already occupied by another aircraft is illegal, except where this is authorised by an air traffic controller. Notwithstanding the inherent risks (where full ATC is not provided) pilots who feel it is a convenient way of saving time (and can't be bothered going around) are leaving themselves open to prosecution.
Here are the relevant excerpts from the ANO:
(2) Subject to paragraph (5), a flying machine or glider shall not land on a runway at an aerodrome if there are other aircraft on the runway.
(5) Paragraphs (2) and (4) shall not apply if the air traffic control unit at the aerodrome otherwise authorises the flying machine or glider.