What would happen if Queen Elizabeth II was somehow rejuvenated?

What would happen if the Queen somehow became rejuvenated and ended up stuck in her mid-twenties?

Read the story below at your own risk. It's not part of the question; just a bonus.

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A mad scientist discovers the secret for eternal youth. But there's a catch: for this to work, the victim must be both very old and still healthy enough to walk on his/her own.

His magical potion must be applied directly to the forehead, from where it will act all over the victim's body and divide his/her physical age by π.

At first, he ponders whether it would be worth it trying to sell the potion and make millions through it. But he doesn't want all this publicity. Besides, most people don't deserve such a blessing. Yes, he's a control freak.

He walks around his old English town in order to buy some groceries. He instead stops at a pub, sees a lot of ugly faces, and becomes reassured that putting his discovery to market is an awful idea. He downs a pint of Hobgoblin and goes back to his lab.

On the way there, he sees a magazine's front page, with Prince Charles in it. He explodes in rage, but quickly he realizes that he's holding the key that will forever prevent this clown from ever becoming King.

By whatever means. Impersonate the press, infiltrate the parliament, anything. Apply the potion directly over Queen Elizabeth II's head. Yell " BEHOLD! LONG LIVE THE QUEEN! "

He's detained, but is allowed long enough inside to watch as the Queen's wrinkles fade away, her hair darkened, her silhouette narrowed around the waist, until she looked as young as in Coronation Day.

Upon seeing the result of his work, the Mad Scientist says “ My work here is done. I plead guilty. Send me to the gallows. ”

The Queen instead ordered the scientist's immediate release, but the Mad Scientist replied. " No. Your fate is to reign alone forever. No one else deserves eternal youth. Not your husband, not your son. You will be the last Monarch of the United Kingdom. If you abdicate or somehow manage to get killed, your kingdom will go down with you. ”