Just trying to picture this and its not adding up. You'll need to correct me if I'm wrong but assumed that you were flying Cologne to Manchester on Hapag LLoyd Express as I only work in ICAO. You have to bear in mind that around London at all levels there are lots of converging routes, and 1000ft separation used at all levels, which I know looks seriously close and can worry those not used to seeing it. I'm guessing you're looking out of the RHS and seeing traffic coming toward you at between 45 and 135 degree plane, you can't see much more than 45 degs out of a window. Near London you will still be in the cruise so at the speeds you'll be travelling if you "saw the crew in the flightdeck" then I'd expect you to be dead now. TCAS would have activated over half a mile away if it actually did. With all the "knitting" that goes on in the London Upper Sector I suggest you were given a turn and descent at the same time for streaming into Manchester with a speed restriction too hence all the noise, and the turn with descent will just compound the feeling of a high velocity manouvre. I'd say the presence of an aircraft in safe close proximity was just the luck of timing.
Please correct my initial guesswork if I'm wrong and I'll try to think of any other non-life threatening possibilities as there are several more that can explain what happenned.