I used to work for Eclipse Internet (a competitor of Zen) and still use Eclipse now I am retired. Zen, along with Eclipse (click here) are in the top five ISPs in the UK. I would expect the sort of comments about Zen that you have already received. You can only improve on them by going to Eclipse Internet!
p.s. If you opt for Eclipse, please put 'Giant' in the referer code - it will help my pension!
We have been with Zen for well over two years and although more expensive than a lot of ISPs we can't fault them. During that time we have had two BT line problems and each time Zen have helped to sort it out. Phone calls to them (Zen) are local rate.
In the main I agree,they are good with support and are in this country. However,like other providers they depend on BT being helpful,and during recent problems in my local area, they could not find out what the problem was. As with other ISP's it is always assumed that the problem is at your end, and they are unwilling, or unable to contact BT to see if they have a problem. On balance,I think they are in the top rank.