mATX cases:
Silverstone TJ08-E
Silverstone FT03
Lian Li PC-V351B
You're unlikely to find an mATX case that will have a side window, and you're unlikely to find a case full stop with a red and black colour scheme paint wise, although quite a few will probably be black with red LED fans, but then again you could put red led fans in any case..
For £100 you could get some cracking cases that will happily take an mATX board though...
Silverstone Raven RV03 is always near the top of my list.
Corsair 500R
CoolerMaster 690ii Advanced
Coolermaster Haf 932
Also, come on Stulid, whilst admittedly it is black and red, there are so many better cases out there, even in the price bracket of the Sharkoon... (CM 690ii Lite) It's also a real shame the K62-R1 is basically impossible to buy. That really was a cracking case with a black and red theme.
Could go with a Bitfenix shinobi, and get the red mesh strips and badge as an option I guess though.
kd