Large zeppelin shaped craft moves many miles effortlessly across London sky then disappears.

Driving away from my flat in South London, my friend and I were sharing a joke as we turned the corner near the park. Up in the sky was what looked like a huge zeppelin with modern windows and lights. There was light cloud and it looked very high up (approx. 5000 ft) and approx. 1/2 a mile or 1 km ahead of us. I stopped the car and both my friend and I were surprised by the scale and luminence of the craft though there was no real detail we could see as it was very high above us. I said it could be a hoax by Richard Branson or someone else but then it moved from the spot it was in to a different spot in the sky to our east and at least 5 miles away as if nothing had happened - really quite effortlessly and in a split second, as if it was floating or moving gracefully, but suddenly miles away. We both felt we had witnessed something quite strange and I scoured the next days papers for any mention but there was none. Funnily enough, the same friend and I witnessed a fireball only two weeks later, also in South London.
Date is approximate.  PD