I have asthma and registered at the GP with no problem. My MIL went with me. I had to fill out some forms and that was about it. I had my medical records with me from the US and gave those to the GP so they had some history on me. I made an appointment for a few days later to meet with the doctor and get a new prescription for my inhaler, again with no problems.
My advice: get all medical records that you can before you leave- including your general doctor, any throid specialits, OBGYN, etc and give all of them to the GP when you register. You should have no problems registering, especially if you're registering at the GP your husband already goes to (ie- there's someone to vouch for you in case there may be an issue- I know there are stories on UKY). Then when you fill out the forms make an appointment to meet with your doctor to get new prescriptions.
As far as the prescriptions for free, you may have to file additional forms for those. I know I was given forms HC1 by my pharmacist. However, I have heard stories about doctors authorizing free prescriptions too. Not sure exactly how it works, but your doctor or the office staff at the GP should be able to help.
And best of luck with the thyroid condition, my sister had hyper- then radioactive iodine treatment and now has hypo. I know it can be a real pain of a condition, and am sending you nothing but well wishes!