I wouldn't consider it "unplayable" even with enemies on a lower latency, but certainly a nuisance especially where competitive play is concerned. The thing such a hike in latency does for me is make my game streaming (from my PC) experience degrade because the input delay becomes more noticeable.
I don't think they will be able to do much since as you pretty much guessed before, it's load balanced. Probably a few other variables that might be at play here, too. All you can do with them is just ask.
I'd say you might have to look at moving provider if you want a better/consistent latency. I made the mistake of going in blind for Zen when it comes to their network topography. Given Plusnet, TalkTalk, Sky, AAISP (TTB?) and Pulse8 (TTB) have better latencies here than Zen, even with some of those services being provided on ADSLx(!), it's just a downgrade to my experience overall.