4 black unmarked police Range Rovers on the M5 - Royalty? Gov. Minister?

That sounds like the Diplomatic Protection Group (DPG) of the Met Police. It was most likely to have been some kind of VIP, A minister or the primeminister. Could have even been the queen or a member of the royal family. The DPG normally escort these type of people round the country they dont seem to trust the local police.......!