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.