With addresses there is always the risk that you send to the wrong one, or it becomes out of date after you send, it's part of the hobby i'm afraid. The best way is to see if there is any recent feedback and send to that address. Or if there is an agency who lists their talent online, look at their website to see if they are currently there.

http://www.royalmail.com/pricefinder has a postal calculator which should give you the price you need.

For airmail letters you can buy stamps with the exact value, or 1st & 2nd class (£0.60/£0.50) which I would recommend because they work like US Forever stamps so price rises are less of a worry.

I used the search, but still have some questions. I am wantingto ship to Jonas Armstrong, but how do i know the address is accurate? There seems to be different ones or they change.

Also I know ircs aren't available in the USA anymore , so I'll have to use stamps so I'll have to buy UK stamps. How many should I use? The royal mail site has about a million options, so it's a little confusing.