My favourite small form factor PC case is down to a low price on Amazon UK
The Cooler Master NR200P is easy, cool and capacious.
The Cooler Master NR200 is a brilliant small form factor PC case - and now it's heavily discounted at Amazon UK. This is a Mini ITX case which allows 360-degree access to the components therein, with space for a 240mm AiO or 155mm high air cooler, triple-slot graphics cards up to 330mm in length and seven fans in total. It's often as high as £94, but for a little while now it's been available for £70 - a great price that makes it well worth recommending to you, dear RPS reader.
Sadly, this isn't the slightly better NR200P version, which adds on a riser cable for vertical GPU mounting, a tempered glass side panel and an extra slim 120mm fan. However, I genuinely think that the mesh side panel and a horizontal GPU mount allowed for with the standard NR200 is the better option for thermals and noise anyway, so if you're not building for aesthetics specifically I'd say that it's not a huge deal.
I've built in two of these cases before - actually an NR200P and NR200P Max, but who's counting? - and they're brilliant experiences. You can pop off almost all of the panels without a screwdriver, which makes mounting components and routing cables quite straightforward. Choosing a Mini ITX motherboard, SFX power supply and 240mm AiO is the most convenient option and makes for a quick and convenient build, but you can make up other setups work with larger motherboards, power supplies etc if you're willing to sacrifice elsewhere. People have even done full custom loops in these cases, which is pretty neat.
Once built, the ventilation throughout makes for fairly cool and quiet components, though of course larger cases are still superior when it comes to overall thermals. If you want something that you can easily pop on a desk or even stuff in a backpack though, the Cooler Master NR200 is one of my all-time favourite options.