The uniform of Major Malloy is wholly incorrect. The Royal Marine (RM) Lovat uniform has dulled buttons and is worn with the green beret, rather than the peaked cap. The crown rank insignia is incorrect and the uniform is missing the RM flashes and the globe devices. Finally, even an experienced marine would be unlikely to have so many medal ribbons.
The Royal Navy ship seen is not a type used by the Royal Navy, but is a US Navy destroyer.
Whilst it is correct that the Union Jack would be flown at the bow when docked, a second one would not be flown from the masthead amidships, as a later shot shows - instead a White Ensign would be flown at the stern. Once under way, the Union Jack would be removed and the White Ensign transferred to the masthead.
Major Malloy's uniform is not British Royal Marine. The uniform used is reminiscent of WW2 United States Army (i.e.: brown) RM service dress is Green Lovat with bronze Badges and buttons. Parade Dress - Dark Blue with peaked cap of white with a red band. The crown insignia used on Major Malloy's shoulder epaulets is not of British type, but resembles that used by officers of the Jordan military, but not majors.
When Ziva and Tony are waiting for the Major, Tony says "what is it about the brits? Jagger, Bono, Beatles, Bond". Bono is not British, he is from the Republic of Ireland, which is not part of Britain. Britain is made up of the UK and Northern Ireland.
The fictitious Drake Hotel shows up on McGee's computer as "1400 Constitution Av. SW". Constitution Av. doesn't have any south west addresses, it runs east-west, from NE to NW.