2. To get the best deal you need to sign up with line rental what disadvantages does this have moving away from BT?
Thinking specifically about your question about
disadvantages of line rental with Plusnet, I can only think of three from personal experience - which all relate to if you have a fault:
1. If you need to report or follow up a fault on your telephone you will have to know your username and password to talk to customer services. This could be a problem if you want someone else to report you line out of order. My previous experience with BT is they only needed to know your number.
2. You cannot do an online fault test like BT provide.
3. You cannot get a free divert to e.g. your mobile phone while your line is down.
All this relates to my own experience of over a year ago, (and I am neither a current BT nor Plusnet home phone customer) so if any of the above concern you you need to follow up direct with PN to establish current practice.
There are of course many advantages, but you didn't ask for them.
Good luck