I've been with Zen for 7 or 8 years. Went with them after looking for a decent ISP once Be Un Limited / BeThere got bought over and folded into Sky's customer base (1).
They've been pretty decent- rolled me onto an IPv6 beta years ago, followed up on a latency issue I was having. It's mostly been pretty hands-off, particularly since moving to FTTC.
I'd love to stay with them, but they don't offer it in my area yet (as
I've been asking about in my thread in these fora). From your perspective, I'm happy enough that I've been "one more month"-ing with Zen since about December 2021 rather than jump to a CityFibre package with another provider, even though I really want a symmetric connection.
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(1): As an aside, Sky folding in BeThere customers -- who went to a lesser-known provider aimed at technical users who want to fiddle with line profiles for greater speed -- to their userbase seemed asinine to me. Sky's config is/was well known to sacrifice throughput for stability, and their support was, um, not great. I'm not still bitter...