Hi Andy,
When I surveyed this, I noted that there was signage for all three of the armed forces as tagged.
I don't have any photos retained, but Bing Streetside (and Google Streetview) shows signs for the air force and army at the enterance. (maybe the navy was a temporary sign?)
My understanding that this was the most appropriate tagging, unless there is a single tag that covers multiple elements of the MoD?
Either way, I haven't been able to confirm online that the navy is present, so I'll ether need to remove that after your response, or take a look next time I'm passing (which may be a while).