Argent, there are some men's clothes shops in the UK (Burtons, Topman, Moss etc). But, to start off on this clothes-buying thing, I recommend Primark, particularly if you live near a large branch (the Oxford Street one is my second home). They offer jeans from a 28" waist (and don't forget that chaps wear their jeans lower on their hips than do women so that is small enough for the tiniest female form). And I have found that I can take handfuls of their men's clothes into the women's fitting rooms (I don't pass) and no one bats an eyelid. And, being so cheap, you can make a few mistakes as you search for your proper look.
And Primark do "boy shorts" in their female underwear section. A way to wear man-styled undies that would pass your mother's scrutiny, perhaps?