Your Dr will not be able to tell your I number.
The following is the advice as per the Inland Revenue website:
What if I have already been given a National Insurance number but I cannot remember what it is?
If you think you already have a National Insurance number but cannot remember it, see if you can find it on any official papers you may have at home. Look on any of the following:
An end of year statement of tax and NICs paid (P60)
Payslip, recent or old
Official correspondence
Annual tax return
Sub contractor’s tax certificate (CIS6)
Employer’s wage records.
Your National Insurance number will not change. Even if for example, you go abroad, marry or change your name, your National Insurance number will stay the same.
If you still cannot find your National Insurance number, ask at your nearest DWP office, Social Security office in Northern Ireland or Inland Revenue office and they will tell you what to do.
I know that the Inland Revenue website is
www.inlandrevenue.gov.uk
Hope this helps
Kathy