It depends on the weight of the envelope.
There are three sizes/classes of letter: letter, large letter and packet. An A5 envelope - technically called a C5 - should go at letter rate provided it's thin enough to fit through the slot on the the size gauge on the post office counter. Otherwise it'll need to go on one of the other tariffs, depending on which slot it will or will not fit though.
See here for sizes:
http://www.royalmail.com/portal/rm/jump2?catId =400105&mediaId=47500706
Now, a first class stamp is currently worth 36p and, at letter rate, will allow you to post up to 100g in weight. Large letters and packets have higher minimum rates for your 100g.
Pricing chart here: ftp://ftp.royalmail.com/Downloads/public/ctf/rm/tariff2008/UK_Postal_Prices_08.pdf
Value of contents has nothing to do with what you're charged, unless you opt for special or recorded delivery etc.