Hmm, well, the new format of NHS number was introduced in 1995, becoming mandatory in 1997, so it is possible for an 18-year-old UK-born citizen to have one.
Did they double-check your date of birth and personal details in the system to make sure it was definitely your number and not a mix-up with someone else?
If you've never registered with a GP before or gone to a GP as a visitor in the UK, then there should be no way you could already have an NHS number in the system, as they are only assigned when you register with a GP.
Also, they should have registered you as a full patient because with a spousal visa, and having paid the IHS surcharge, you are entitled to full NHS care from the day you arrive in the UK.
You should only have to register as a temporary patient if you will only be staying in the area for between 24 hours and 3 months... but you have moved permanently, so you are not classed as a visitor.
This is the official guidance from the NHS on registering new patients:
GENERAL GUIDANCE ON ELIGIBILITY TO FREE NHS PRIMARY CARE
1. Who is eligible?
Free NHS care is generally for those ‘Living Lawfully’ within the UK and considered to be ‘Ordinarily Resident’ in this country i.e. here for ‘Permanent’ settlement or ‘Habitually’ residing here. It is not related to one’s Nationality, how much Tax or National Insurance he/she has paid in the past. Most important: One must establish what commitment the new arrival has made that confirms his/her permanent settlement in the UK. Another point to confirm is what the individual is intending to do here for the next 6 to 12 months i.e. employment, studies, renting accommodation etc. The answer would confirm as to whether he/she can be considered as ‘Ordinarily Resident’ in the UK.
Note: The onus is always on the patient to prove their eligibility to free NHS care.
2. How do you decide?
- Living Lawfully –To confirm ‘Legal Status’ – Request Passport, check visa and its validity dates, European Nationals may only produce an ID card. This is acceptable and will confirm whether one is legally in the UK. Recent arrivals from EEU only travel with an ID card and no passport. Ensure ID photo matches the person in front of you. (EEU Rules: see last page)
- Ordinarily Resident - Confirming or determining a persons ‘settled status’ is key.
For the purposes of the NHS if a new arrival provides documentary evidence that confirms he/she has been or intends to be in the UK for a particular purpose for 6 months or more i.e. Work Permit for a fixed period and tied to a particular post, being a full time student, Spouse joining the other half for settlement as a dependant, Asylum Seekers, Dependents of an NHS Patient etc. In such cases you must provide full registration as they are eligible to free NHS care from the moment they arrive in the UK.
Please note that a newly arrived individual does not have to first live here for 6 months before NHS care can be provided– practices are in error when refusing to register using this reasoning.