120Hz is amazing (I have one), but is currently limited to 1920x1080, which I personally wouldn't want in a 27" monitor (I would want a higher resolution, and thus smaller pixel size). Furthermore, unless you attempt to overclock the monitor, you will be limited to TN displays.
Is your primary interest color, black levels, viewing angles, etc. or in very smooth movement and higher framerates?
It doesn't have to be 120Hz, I just know that can make gaming smoother. Budget, I guess I would have to say £300-£400. I would like a monitor with at least 1 HDMI port. As for resolution, whatever will make the games i play look best. I plan on playing battlefield 4 when it comes out.
If you are willing to step down to a 24" (which it seems you would need to do in order to stay in you budget), a lot more options open up to you. The majority of 120Hz (and 144Hz) monitors are in this size range as well as a lot of other good options.
For arguments sake, let's say I chose to stay at 24" and I wanted 120Hz as a primary feature, staying also with HMDI included, what would be my options then. Same price range. P.s. sorry I'm so useless at this kinda thing, I just want best value for money.
With a GTX 770 I would strongly recommend going with a better resolution (or higher definition) than 1080p and upgrading to 2560x1440 (1440p).
You can get some very affordable 1440p displays and some of them can be overclocked to to 120Hz, giving you the best of both worlds (1440p AND 120Hz). Gaming in 1440p will ruin 1080p for you the same way 1080p ruined your old 480i TV.
The Qnix is an example of a 1440p monitor that can be OC'd to 120Hz for 350$ and is the best bang for your buck you can get.
cookiebiscuit has a point, especially since you have an nVidia card. However, if you don't want to wait for G-Sync monitors to come out, you could get a current monitor, and later add a module to enable G-Sync whenever they come out. Current ones that are good and in your price range are:
If you go with Pete_the_Puma's route, be aware that Qnix is a Korean brand (maybe questionable quality) and is very limited. (Only DVI) Furthermore, while many of the limited Korean 27" 1440p monitors can be overclocked to 120Hz, some can't reach that high. It depends on the individual panel. There may also be some strange customs costs, seeing as your in the UK.
Quote from the article 722C posted that sums up my feelings:
"The only problem I face is that using a smaller/low-res monitor ever again will likely prove extremely frustrating after such glory."
Sure there is SOME risk involved, but getting the screen through Amazon I believe offers additional insurance/protection. I ordered one from Korea through eBay and the process was a breeze, the monitor is phenomenal.
For the Qnix, quality is subjective, the panel is made by Samsung, so no complaints there. Where quality is obviously lacking is stuff like the stand, which is super wobbly and has no adjustments, but it does the job. It also has crap speakers, but I don't think anyone actually uses monitor speakers. For me, its pretty much the perfect monitor. Cheap, 1440p, excellent colours, 96Hz, input lag of a TN display thanks to single input.
Ordering from Korea is pretty stressy though, if you get a bad one they take it back but it would be a headache.