Zen come highly recommended. £28.00 per month, but no 12 month contract to sign. Gamers like ZEN for it's speed and reliability, even the users forum is full of praise, you have to search hard for any serious complaints.
Most people I have had contact with say that activation was on time or 1 to 2 days early. j.
Have a look at PlusNet - £21.99 pm They keep their customers fully informed when problems arise, and their support people I have found very helpful. Also have a look at the PlusNet discussion forum on the Adslguide website. Been with them for 3 months and have no complaints.
i use force 9 plus nets sister company and it is the best isp i have ever used, as Nosmas said if there are any problems you just post in their help room and a member of their team get back to you for free with all the gen you need. I was set up within 4 days!