Change Route: A405 (M1)

21A is an odd number for this junction because it isn't newer than the others - it was built at the same time as the motorway it's on. Its sliproads are interlinked with those of junction 21 so presumably the M25's designers thought a separate number would be unsuitable. In any case, brace yourself for junction 21, it's coming up just moments after this exit, and it's a big one.