They'll probably exceed your expectations nesty. Plus, at £20 for a pair of walking shoes, even if you had to buy 3 pairs over the course of a couple of years, you'd still be saving money over one more expensive pair.
I paid £75 for my current Meindl shoes. They've lasted me 2 and a bit years. That's slightly longer than my old Hi Tec walking boots which I paid £45 for and half as long as my first pair of Peter Storm boots which I paid £10 for. All have been waterproof and grippy. Granted, I don't regularly do mountains (not many of those in East Yorkshire
) but the terrain I do walk on is stony and muddy so, to me, same difference but without the gradient. I think any walking boot will cope with an occasional foray up a mountain anyway. Winter mountaineering is a different kettle of fish though.