Over £50 isn't excessive, but considering the budget it's probably a good limit.
Fractal Design Core 1000 mATX case - £29
I honestly wouldn't go quite this low unless you really want to scrape by with the bare minimum. The (optional) rear fan isn't even 120mm. This is however likely to be the best-built case at this price (might be worth moving the fan from the front to the side panel).
Fractal Design Core 3000 ATX case - £45
The Core 1000's bigger brother - way better equipped. Arguably the best deal at this price. Cable management, removable drive-bays, three fans included (two of which are 140mm). The only problem? USB 2.0. Not a big deal unless you're transferring large files a lot?
Antec One - £45
Good alternative. 2x120mm fans and cable management holes, plus USB 3.0. Not quite as flexible though.
Corsair 200R - £50
Similar to the Antec One (2x120mm, USB 3.0 and cable management), but with the Core 3000's flexibility.
Silverstone PS07B - £55
A bit of a wild card. An inverted mATX case with the usual cable management, 2x120mm fans and USB 3.0. This'll should be a step above the other cases and it has the benefit of being small. On the downside, it's small, so it'll be harder to build in. You'll also be restricted to mATX/ITX builds in future, but I figured I'd put it forward since it's an interesting case at a good price.
Honestly, you'd probably be content with any of these, with the exception of the Core 1000. If you aren't really sure which to go with, just pick whichever you like the look of best.
Edit: I totally ignored the monitor question. With the case and the optical drive (about £12), I don't think you have enough to squeeze a monitor in there except possibly for the cheapest 1360x768 monitor going.