My point: if your SERVER is in the UK, but your CARRIER is not, the call goes to the server and back to the carrier and back to the uk. Nothing good can come of this.
You need a carrier closer to YOU or a carrier closer to the UK, moving the server to the UK does not connect anything with the uk, but does pass a signal through it.
it's like driving through a town on a superhighway, but finding out there's no EXIT in that town.
you don't need to colo in the uk to use a uk carrier.
your best bets: a carrier that handle UK traffic with an IP address near to YOU for low latency OR a carrier IN the UK that handles UK traffic.
if you GET a UK carrier with an IP in the UK, THEN setting up a server in a colo in the UK would be cool because the UK calls would both be local, which would make bridging their calls very low latency.
but that is entire dependent upon having a UK carrier (otherwise the call will still have to travel to the carrier to enter the world of traditional telecommunications)