hello,
You can have a look at the NHS website:
http://www.nhs.uk/
As long as you are legally living in the
UK (permanent resident) you can register to a GP as soon as you arrive in the UK.
You will have to find your GP in your area, as far as I know if you choose one that is to far from where you live they will refuse your application.
In my case I need have a prescription for my asthma (inhaler)so when I ran out of it I have to contact my GP to have another prescription (it takes 2-3 days). I have to go and get it and then I can go to any chemist to have my 2 inhalers. It cost around +- £6 per prescription.
If you need to see specialist, it is usually your GP that will refer you to one.
If it is an emergency of course you go directly to the hospital!
Since I am here, I have not really had good doctors, I can change if I want but I'm too lazy !! My current doctor just like to write prescriptions without really looking at what's wrong. He makes stupid jokes ( a bit sexist I would say) and keep asking me where I am from everytime I go see him !!
If you require to have a lot of prescriptions (for more than 5 prescriptions in 4 months) you can have the "prepayment" option which is cheaper.
hope this helps !!
Joanne