I've just finished "assembling" a PC for my son via parts from SimHQ members and from various eBay sellers and now the last part is to get a new case for him. At the moment, what I got is an old beige tower case, the one with only a fan at the back. I am thinking of giving him my Antec 300 case and thus upgrading my own.

I am looking for a good, cheap, aircool case. I subscribe to the "cool air in from the front/bottom/side and out the top/back" method of cooling and my current Antec 300 has fans blowing out the top and back. I don't have any intake fans and though my side-fan is mounted, I've never had the need to turn it on as my GPU and CPU temps are alright. My only complaint about this case is the lack of cable management, plus with my new 6970, is a very tight fit.

So I'm looking for a case, hopefully under Ł100, with a proper airflow layout (none of that meshes-everywhere-and-you-don't-know-if-the-air-is-supposed-to-go-in-or-out cases) and cable management. So far, a CoolerMaster HAF 912 looks good, but still searching for other options.

Well, the HAF 912 ("Plus" version, I presume?) is a logical choice. I used one for a recent donor system. But if you want an option, the "Arc Mini" from Fractal Design is a nice clean looking case.

I really like my Fractal design R3, understated, stylish, quiet and has the flexiblity to add more fans if necessary, including the configuration you describe. In fact I pretty much have the same config as you describe, except I have an extra fan in the side panel to keep the gfx cards cool. I decided it would be a good idea after adding a second 460.
Cable management is quite good as well with enough room behind the mobo tray to hide most of the cables.

Current favorite.
106 Euros

http://www.corsair.com/pc-cases/carbide-series-pc-case/carbide-series-500r-mid-tower-case.html

HAF922

I use the same and love it

HAF 922 does it all -- I have 3 up-and-running in the house.

BTW, the 932 is bigger plus one fan plus USB3 on front panel. Everything fits in the 922 and its cheaper even if one adds the fan. Either is a great long term case -- a mini-investment.

Yep, the HAF 912 is currently the best option I am seeing at the moment -- particularly wins in the price department. The other recommendations are nice too, like the Carbide 500 and the Fractal R3. The HAF 922 and 932 are a bit on the pricey side for me, and the extra fans aren't really a "need" for me right now since my temps seem to be under control anyway.

I guess all I really need right now is additional space and cable management... with my two fans (top and back) and passive air inflow, it seems to be enough to keep my system cool so not looking for a case with fans everywhere.

What I find funny is the placement of dust filters... most decent cases have front and/or bottom dust filters, but how come the side-fan doesn't usually have one? Even with higher-end cases that have dust filters at the top fans, the side-fan doesn't have any...

Very happy with my Rosewill Challenger. I did add more fans and upgrade the ones that it came with, and added a fan controller.

http://www.newegg.com/Product/Product.aspx?Item=N82E16811147153&Tpk=Rosewill%20CHALLENGER

A bargain at $55.

Don't seem to have Rosewill here that I can see....