Once meant to be part of a long-distance south-coast motorway, the M27 only really manages to bypass Southampton. It is constricted by the New Forest in the west, whose guardians the Verderers refuse to allow anything much to be done about the A31; and in the east by - er, nothing. The A27 continues from its eastern end as a four-lane motorway standard road, and continues with various motorway standard sections all the way out beyond Brighton. For no real reason, this remains A27.
Very early 1930s plans for a national network suggested a motorway from Ramsgate to Penzance: while this maybe isn't needed, extensions to Brighton and Bournemouth would definitely come in handy.
The M27 forms probably the best terminus of any London radial motorway. The M3 comes via Basingstoke and Winchester, but is stopped from crashing headlong into Southampton by the M27 which spreads its traffic across the urban area, preventing either a sudden crash into an urban area or a spluttering death on a lower standard of road. Good work.
With thanks to Robert Bollen, Nigel Le Poidevin, Steve, Dave McGuire, Clive and James Denson for information in this section.