Watch the Tones & Gear online guitar lesson by Angus Clark from Focus On: British Metal
It's a fairly simple formula: Humbucker guitar into a medium gain tube amp with some kind of pedal in between. The pedal would be a Treble Booster for Tony Iommi or Richie Blackmore, it'd be a Tube Screamer for Thin Lizzy and forward. And/or a wah wah. The tube amp is most likely a Marshall, but don't forget Laney and Orange. The neat thing about the British bands is that they were not getting the modded Marshalls that all the California guys were getting, they were putting pedals in front of the stock amps.
Blackmore is unique because he used the mic pre in a reel to reel tape machine between his guitar and amp - which gave him a very smooth compression. There are lots of pedals out there to emulate this kind of thing now.
The humbucker guitar is very often a Gibson, but those guys would put a PAF in just about anything, Tipton and Dave Murray both had strats with humbuckers in them. Blackmore, and Fast Eddie Clark from Motorhead are the most notable Strat abusers in the scene, and their contribution cannot be underestimated.
I'm using a Gibson USA '61 reissue SG with their classic 57 PAFs in it. It is not a high output guitar. The rhythm tracks were cut using an '85 Marshall JCM 800 with an SM57 dead center on the dust cap. The live stuff for the video is done through an 11 rack using maybe the Soldano model? Remember the tone is in my fingers so please don't ask me for my settings, I'll get offended.