http://www.royalmail.com is the British mail site, you can find out postage details on there.
It depends which 1st class stamps you have, small or large (both gold, but the large ones helpfully say "large" on them). You might have some 1st class decorated ones, like special Christmas ones or whatever, these are fine.
1st class stamps for small letters (max 240mm x 165mm x 5mm, 100g) are 36p (they went up this year).
1st class stamps for large letters (max 353mm x 250mm x 25mm) are 52p. There are different large ones for different weights, but the ones you usually see are for max 100g letter, which are the 52p ones.