Those parts of Greater London, Bedfordshire, Buckinghamshire, Cambridgeshire, Hertfordshire and Oxfordshire bounded by:

On the North by A421 and A422 from Bicester to Old Stratford, the A508 to junction with Grafton Way, a straight line to the A509 / A422 junction. A422, A428 to Bedford, the B660, B645, B661, and A1 TO A14 / A1 junction at Brampton, A14 to A14 / A141 junction at Huntingdon. A141, A1123 to the A10 at Stretham.  On the East by A10 to A14 at Milton, A1309 via Cambridge to A10/M11 (junction 11), the A10 to Royston, A10 to Ware, A1170, B176 to Cheshunt, the A1010 to Tottenham and A10 to the Thames at London Bridge. On the South the Thames to Westminster Bridge. On the West the A41 bypassing Watford, Hemel Hempstead and Tring to Aylesbury. The A41, A421, A4421 at Bicester.