Naval Base, Devonport.

This is an operational base, so you can’t just turn up and have a look round! However there are a couple of ways of getting to see parts of it. One way is to apply for a place on the frequent guided tour (twice daily, several days a week). This could include a submarine or warship visit. They’ll let you know if your application is successful. You might want to enquire what you should bring with you, to avoid a panic attack on the day on the lines of “where’s my photo ID?!” I can’t tell you if they’ll run a security check on you or not…
It would be a good idea to check beforehand the location of the meeting point and how to get there. Devonport is a big place… Once you have located the meeting-point on the day, you should be able to park in front of the reception centre, assuming the armed guards let you in.
Another way of getting in is to wait for one of the Navy Days or Heritage Days, when you can just turn up and look at various museums, and go on timed minibus tours of parts of the site. This might include a submarine or warship visit, and historic buildings.
A note of caution: I don’t mean to be rude to the overweight and disabled, but for visiting warships and submarines, a certain standard of agility and fitness is required. Operational vessels have steep ladders and confined openings.