Be aware that Royal Mail has a 2kg weight limit. Over that it has to be sent with a courier.
First Class mail will get there overnight in a lot of cases. As long as you send it recorded you shouldn't have a problem. I've never found the need to go the extra mile and use special delivery, the prices are already so expensive.
I think you'll find that shipping in the UK is far more expensive than in the US. Plus, you'll be charged VAT on everything in Europe. Shipping to the US, Far East and Australia will set you back a minimum of 9GBP.
You might want to look into getting a franking machine... Unfortunately, there's no programs like Stamps.com here. Most of my packages have at least 5 stickers on!!