I've been with Unchained for a year and they are by far the most reliable and performant Internet service I've ever had. The FTTP ordering and install was excellent, as was the support with initial router configuration and connection. Sessions remain solid for months at a time with all stats available in their customer portal, far superior to that of my previous tv-bundled broadband package.
Customer support is amazing: I think the tickets I've raised have been responded to, usually by Keith, in about 10 minutes.
All this at very reasonable price for a package that includes a static IP on a reasonable contract term.
If you rely on high quality, stable Internet access for work (like I do) then Unchained are definitely the way to go.