In that case, you can apply to the General Register Office for your grandad's birth certificate. Go to this site:
http://www.gro.gov.uk/gro/content/certificate/ index.asp
Follow their directions and fill in an online form. You will need to give them the index reference for the birth registration. To find this, there are a number of free sites you could use to search for it. One of these is FreeBMD, although their records might not be complete. Another site is ancestry.co.uk. To search their records, you will need to sign up for a 14-days free trial. If you do, be aware that it is a deferred subscription, so if you don't wish to sign-up after your trial has ended, you will need to contact them to cancel.
Of course, you can also search the national birth registration index at your local records office.