Pawe³ Piotr Stawski wrote:
what is the most important key motorway in Great Britain?

I'd have to say the M25 -- traffic from the continent to most of the
country will use it. The M20 has an alternative route (A2/M2), but the
M25 (especially if you include the dartford crossing) is pretty
unavoidable (A25 through Redhill etc? Blackwall Tunnel!? A26
Tonbridge/Tunbridge Wells?)

The M6 A52-A556(M) is important, however traffic from Manchester could
take the M62/M1, A42 to Brum. The M62 over the Pennines is probably
next, there aren't many cross-pennine routes. Scotland only has 2 roads
into the rest of the country/world of course, so they are pretty
important too, but if you lose the M25, the A14 is your only real hope.