April 21 is Queen Elizabeth's ACTUAL birthday. She was born April 21, 1926. Royalty that are born in winter or poor weather months have an OFFICIAL birthday in the summer, a Saturday in June. Both are celebrated every year. Official celebrations are held in summer when there is a greater likelihood of good weather for the Birthday Parade, also knows as the Trooping the Colour. I'll post about that when it happens!
(She wasn't actually there. The Queen takes a week of from her royal duties for a week and has spent her actual birthdays quietly at Windsor Castle with her family and close friends for the past 4 years. At 89, I don't blame her!)
The King's Troop Royal Horse Artillery take a route down the Mall to the Tower where they would shoot a 61-gun salute.
Central London, and lots of other places, is full of smaller alleys and back roads of shops or restaurants that are pedestrian-only or at least pedestrian friendly.
Happy Birthday (kinda) your Royal Highness!