I have a lot of questions but the one which is taxing me at the moment is should I change my btinternet.com email to a Icloud email to make sure that the functions are seamless with my IPhone 4s?
If you're using IMAP on a PC with that email provider, it will work exactly the same if you set up the Mac email client to use IMAP with that email provider... you should notice no differences in how it works. Setting up iCloud and how you want to use it is entirely up to you... if you want it to replace your present provider, you can...nothing says you have to.