A £15 case is what $30-$45 us dollars? A case that inexpensive probably isn't worth your time and won't be an upgrade unless you manage to get a stunning deal on a used case or a model closeout. What I usually recommend for an entry level case with good cooling that can be found inexpensively is a
Cooler Master CM-690 III The HAF series will move more air, but are generally more expensive. Thermaltake cases are also quite good but tend to be a bit more expensive for the same quality, and tend to be a bit flashier than I tend to personally prefer. If you want a good overview of all the mid to high end cases out there, with pictures I recommend taking a day to look through the case listings here: http://www.performance-pcs.com/catalog/index.php?main_page=index&cPath=103&zenid=75867c17b6051c5df5b5373eec586483
If you can find one, the holy grail of cases remains the
Cooler Master ATCS 840. They are expensive, there never were that many of them to begin with, and they aren't making any more because of the price point, but you won't find a better case. Period.