Hi Giovanna, we moved here from the US a couple of years ago and found the medical system to be quite different. As per previous posters, you will need to register with a GP upon arrival (have heard good things about Thurleigh Rd surgery, as GP practices as called here); visits -and usually medication- are free under the age of 16. Pediatricians are considered consultants (specialists) and need to be referred by GP.
Having said that, you also have the option of registering with a private GP which you pay for and will have usually faster access to specialists if you have private insurance.
Good luck with the move and please PM me any questions I could help with. You have chosen a great area to settle in.