I seem to remember if you migrate an existing FTTC service to Zen you will just be on a 1Month contract as BT Openreach do a move not a cease and re-provide. (It's Openreach that force a 12month min contract on the IPS's) I am not sure if this has changed or now.
I like Zen, they are reliable i have been with them Since 2004 and not had any issues, I more recently moved house and got FTTC. I have not found any problems with it.
Some people have been reporting "Slow Single thread speed" Its not something I have noted though. Zen were being quite open about it issue for a while but seem to have gone zip lipped now.
The only long term issue I had over the years was a outage of a few days and that was because of a major BT outage.
Zen are a lot better when they have kit in your exchange if not they have to rely on BT backhall which they have very little influence over (this is the same for all ISP's) check on a site like https://www.samknows.com/broadband/exchange_search
Various (Dile up) -> clara.net (Dile up) -> TELE2 (Microwave) -> ZeN (ADSL)