I have two ISP's (two lines!) Zen and PlusNet and from a connection, speed and reliability standpoint they have both been good. I have had the Zen connection for around 10 years, the PlusNet one for just over 2 years.
The stand-out difference between the two (in my opinion) is the customer service, Zen are streets ahead and have always been excellent in this area for me. I like the fact that their helpline rarely has much waiting time, their staff are in the UK, polite and knowledgeable. PlusNet on the other hand are the opposite, if you need to speak to them then be prepared for long waits and "call volume" recorded messages.
If I had to reduce to one line, I would keep Zen and cancel PlusNet
C