Can we clarify, latency to where? I would have thought that an increase in latency to Zen itself would be impossible, no? Is it latency to some further-off server?
It's very possible.
If there's a break in a fibre somewhere then traffic will be routed around the break, increasing latency.
As mentioned by others, unlike most providers who terminate all their customers connections in London, Zen also have an LNS in Manchester.
Most UK based servers are in London so traffic having to go to Manchester then to London and back again adds a fair bit of latency, particularly for anyone in the South of England.
As jelv points out there can sometimes be a noticeable difference in latency between Zens 2 London gateways, with lon1 often (but not always) being lower latency than lon2.
Low Interleaving on VDSL2 also adds a minimum 8ms latency at the DSLAM. High Interleaving adds even more. The lack of a resync rules this out though.
A tracert may give the answer.
Edit: g3uiss replied while I was typing.
