My employer would like a functional abilities form, and other functional questions answered by my GP doctor after being off for over 2 years. I requested they access this information from my LTD insurance provider but said they need this from my GP. Is this correct? Seems strange when my LTD has all the information on me, especially since going through their in depth rehabilitation program.

It certainly is a bit odd, but if they insist on getting the information directly from your GP, then it should be done. It's possible that your LTD file has additional personal information that your employer is not allowed to know, so they don't want to provide that to protect your privacy.
If you run into any questions or issues with your employer or LTD provider, please give me a call asap and we will help(phone number below)