Look up automotive paint suppliers. There has to be something near you in London. I live in a tin-pot town in the sleepy south west of England. We have a place in my town. However they always seem to cop-out of guaranteeing the exact match. The technology is typically meant to match known automotive colours but then tweak the mixtures to age match the paint, as opposed to decoding the makeup of an unknown.
Try Yell.com would be my advice.
Aa above, Any decent automotive paint supplier/ body shop will have the technology to paint scan the paint, some body shops may not paint match unless there getting the job to respray though