A right base join into a left hand circuit is, to my limited knowledge, legal
UK Rules of the Air
Flight in the vicinity of an aerodrome
12 (1) Subject to paragraph (2), a flying machine, glider or airship flying in the vicinity of what the commander of the aircraft knows, or ought reasonably to know, to be an aerodrome shall:
(a) conform to the pattern of traffic formed by other aircraft intending to land at that aerodrome or keep clear of the airspace in which the pattern is formed; and
(b) make all turns to the left unless ground signals otherwise indicate.
(2) Paragraph (1) shall not apply if the air traffic control unit at that aerodrome otherwise authorises.
2 s