From what I have read all NHS doctors prescribe synthetic thyroid meds is that correct? I have also read that none of the private insurance cover pre existing conditions.
That's likely to be because, in the UK, your first point of contact for medical issues will be the NHS, and there's no such thing as a 'pre-existing condition' on the NHS... you will be treated regardless of what condition you have and how long you've had it.
Private insurance is usually only used as a 'top-up'... namely, to skip waiting lists for NHS operations, to get elective surgeries not offered on the NHS, and to get treatment or operations that are considered too expensive to be covered on the NHS (admittedly, though, most things are covered).
Do I find a GP as soon as I get there? I will be on a Spouse visa.
You'll need to get some mail in your name, so that you have proof of address, then you just go along to a GP within your local area who is taking new patients and ask to register
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You can search for local GPs here:
http://www.nhs.uk/Service-Search/GP/LocationSearch/4