I'm almost sure that the reason for you routing via Manchester will be something to do with BT Wholesale!
Zen terminate BT Wholesale traffic in London and Manchester. They also terminate LLU traffic in London and Manchester.
I live around 45 miles north of London. When I was on BT Wholesale, Zen terminated my traffic in Manchester, meaning latency to London had an extra 7-8ms for the trip to and from Manchester. When I was moved to LLU, Zen terminated the traffic in Manchester but more recently they switched to terminating traffic from my line (and presumably all other LLU lines on the same exchange) in London.
Unless it's changed, Zen's routes to the west (Ireland and North America) are lower latency from Manchester than London. This means that my latency to some US destinations increased slightly when my line was moved to termination in London.
London termination will always result in lower latency to London based servers, but it isn't always a good thing. Whilst low latency is nice, it is doubtful whether 7-8ms extra latency makes that much real world difference, especially if it does not add additional jitter.