What is the fastest way to send a letter UK?

Okay, so, fastest way to send a letter in the UK? Right, if I really need something to get there like, yesterday, I'd absolutely, no question, use Royal Mail's Special Delivery Guaranteed by 9am. Seriously, it's a lifesaver. I remember once, I had to send some super important documents – it was for my house purchase, total stress-fest – and I was so panicked about them getting there on time. Special Delivery? Boom. There the next morning before I even had my coffee. That feeling? Priceless. I mean, sure, it's not the cheapest option, obviously more than just chucking a stamp on it. But honestly? When you absolutely, positively need it there by 9am, what's a few extra quid, right? They have a 1pm option too, which is… fine, I guess. But if speed is the main thing, the only thing, then it's gotta be that 9am guarantee. No two ways about it. I wouldn't risk anything else, not when it matters. You know?