Zen are fantastic but their FTTP is expensive.
I get an absolutely cracking deal at £18 a month unlimited FTTC which is about 80 megabit, I have been a Zen customer since 2008 so they gave me a pretty good deal and also put me on a no price increase thing.
I am probably going to upgrade to FTTP as its available, but seems a big jump up, particularly if you only go 100 megabit, id probably just go for 500.
That being said, Mrs pays phone line something like £23 a month, so bin that off and get the £7 a month VOIP thing they do, so you are saving there.