When I arrived at the UK Airport with my Spouse Visa, I was instructed by the Immigration Officer to go to the NHS Office (near the immigration que ). The Head Nurse asked me if I ever had my pap smere check-up within the last 3 months. Luckily, I got one (in UK) before I went back to Philippines, as I stayed in UK with a tourist visa.
I think, they just want to register your name with the NHS. They usually send out letters for clinical purposes like inviting you for pap smeres, etc.
cheers!
