Re your grandfather's military records, PJ, it depends which war.
For WW2, there are the collections mentioned above. However these mostly aren't records about individual personnel.
To get an ancestor's WW2 or recent military record, you generally have to write to MOD: www.gov.uk/get-copy-military-service-records/apply-for-someone-elses-records
For WW1 records, the picture is both better and worse. Many are available online, either directly from the National Archives or Kew has contracted them out to FindMyPast and Ancestry. See here for what's available where:
www.nationalarchives.gov.uk/help-with-your-research/research-guides/?research-category=online&sub-category%5B%5D=first-world-war
Fortunately you can search the commercial sites without a subscription before committing to one where you think you have a hit. Knowing your grandfather's place and date of birth is a good head start, as these are noted in the records and you'll be able to include them in the search boxes.
HOWEVER. Huge swathes of WW1 records are completely missing, due to WW2 bomb damage to the record office. Those which survive are mostly from the Burnt Collection, or are those which had been sent to another office to be analysed (WW1 outcomes were used to predict WW2's likely rates of post-war incapacity and pensions). So if your grandfather was injured in WW1, his service record is more likely to have survived.
The one type of record that survived for pretty much all those who served in WW1 is the medal card (and medal roll, medal list). All those who served in theatres of war were awarded the Victory Medal and British War Medal. There were also other common types of medal: 1914 Star, 1914-15 Star, Silver War Badge (for those injured and discharged). Those records may give pointers as to where your grandfather served, but if he has a common name it can be a thankless task sifting them.
Try this project by livesofthefirstworldwar.iwm.org.uk, which was seeded using the medal cards. Someone might have filled out a bit of his profile, helping you pick him out.