lockyatlrg's profile places him in Derbyshire, which is close to half way between London and Manchester, though probably slightly closer to London. He complains about 20ms round trip time to London compared to <10ms when previously on the London gateways. There are very few applications where an extra 10ms round trip time is a deal breaker, especially if the jitter is low, which is presumably why Zen won't reverse their decision to route his traffic through the Manchester gateways. I'm sceptical that even the best online gamers would spot 10ms higher latency, though RTT is easily measured and it's understandable someone wanting minimum latency to London will get upset about a routing change that increases London RTT. Unfortunately Zen have to operate based on sound network engineering, not minimising London RTT for an individual customer on request.
If minimum London RTT is the key criterion, an ISP with all its gateways in or close to London would be the best choice as routing all traffic through anywhere other than London will increase London RTT. You'd have to be paying a lot more than Zen's consumer FTTC charges to be able to dictate your primary gateway location.