The HSCIC does know about you.
Well, you will certainly have some data in an HSCIC system as they run the GP registration database, so if you have ever been registered with a GP or issued an NHS Number, you will be on the NHAIS and PDS if nothing else.
The NHS Central Register was originally the old national ID system from the second world war - everyone already had a national ID card so they simply reused the old ID numbers as NHS numbers (only renumbering everyone in 1995). It's even run out of the General Register Office.
For staff who have access to this old system to trace people as part of their job (patients have an annoying tendency to move house and not tell you) - for people around when the NHS was founded in 1948 they can see their entire address history and every GP they have been registered with, along with their old second world war ID card number.
Another Big Fun Database is the Secondary Uses Service (SUS). If you have ever set foot in a hospital since 1987 details of that care "episode" (with your NHS number) will have been transferred to the SUS. Even if it's as simple as "patient booked into A&E, but refused to wait and went home". Currently, around 125 million records a year are added to this system, with returns being sent on a monthly basis. Don't worry though; most psychosexual stuff (fetishistic transvestism etc) HIV and IVF records are pseudonymised or anonymised at submission.
The best way to find out what information the HSCIC does hold about you (and the HSCIC does hold information about you, whatever opt-out method you have tried to use) is to complete a Subject Access Request form and ask them to send you a copy of all information (don't bother with the paper record request part of the form, you are only interested in the digital bits). If you GP has screwed up and transferred information without your consent you can then involve the ICO to have the data removed.
You can see a list of many of the national NHS Digital systems at http://systems.hscic.gov.uk/.
Another thing that most people don't realise is that GPs have a contractual requirement to have their entire medical record and administration system hosted by a private company. Around 50% of GPs have their records hosted with EMIS Group plc, around 2,500 practices and 40,000,000 patients are on TPP's SystmOne (the underlying database there is a MS SQL instance of about 700 terabytes) and most of the rest are on INPS Vision.