You're the cat's pajamas simply for being willing to ask the question of just how British you are. While being at least partly British has been a thing that many people in many countries have had to learn somewhat unexpectedly in the past, many people worldwide are just naturally born kinda British.
The best part is, you don't have to be from Britain to be secretly British. You simply have to have a strong sense of fair play, a horror of public displays of emotion, an excessive fondness for beer, and the ability to reflexively say sorry to someone when they're the one who trod on your foot, not the other way around. You may also love dogs, but not be so keen on children, enjoy layering every single outfit you own year-round so it can adapt to just about any weather scenario, and be willing to go on a picnic in the rain because that's the day you planned to picnic and gosh darn it, a little drizzle isn't going to stop you!
By Jove, old fellow, you really are a good egg for deciding once and for all to establish your Britishness - or lamentable lack thereof - using our indisputable, objective, and highly scientific method of answering these questions. Pour yourself a lovely cup of tea and click onward!