If you don't care about 3d, I suggest you not bother with the Samsung LS27A750DS (not sure why Amazon UK adds the extra letters, as far as I can tell, the actual samsung model number is just S27A750D). The T27A550 has component and composite inputs, and even a TV tuner, which are good if you have old VCRs or old video game consoles to connect to it. If you don't have devices that need those inputs, it's probably not worth the extra cost. The BenQ you list is an A-MVA panel which has better static contrast ratio and viewing angles than the others, which are TN panels. However, the TFT Central review of it found it has considerable motion blur and average input lag of 33ms, making it unsuitable for gaming.
Since you give your budget in british pounds, I presume you're in the UK. If you can save up a little more, perhaps you could consider the Hazro HZ27WC, available for £399.92 (inc VAT) here:http://www.scan.co.uk/products/27-hazro-hz27wc-led-ips-pro-glass-version-8bit-dvi-2560x1440-420cd-m2-10001-6ms-speakers-pc-mac
TFT Central showed the motion blur as mild and measured the input lag at 8.8ms, plus it's 2560x1440.