You're not alone! I saw it too whilst on the M25, but posted in the UFOs thread in JetBlast. I reckon at least 2,000 miles away so not really surprising that it appeared stationary. As well as it being the only space mission launch that I've seen, it was also a good reminder of why we use Great Circle routes - California being roughly NW from here.
(Thanks to
treadigraphfor pointing this thread out).