How old are the stamps? Maybe they're worth more than their cost. :-)
The stamps are UNUSED. I don't think there should be a problem with using them. I'm still using stamps which I purchased three years ago and so far haven't had any problem.
Stamps which are shown as 2nd class or 1st class are dateless. They are valid forever. That's the point of them.Royal Mail work on the basis that people will not buy vast hoards of them in the hope of saving money when the postal rates go up !
Because of the hideous above-inflation increase in postal rates recently, buying stamps in advance has proved to be a remarkable cost inflation-buster of an idea. I'm working my way through yet more books of them this Xmas.
All post decimal currency stamps are still valid for postage if unused, either at face value or for 1st/2nd class mail or equivalent postage where there is no face value.