I think you should give CS:S or BF2 a go and see what they're like, and maybe try a movie with some fast action sequences. If after that you decide you're not happy with how the screen is performing, you can look into going down the distance selling regulations route. There is a time limit on that, you have to let them (the retailer) know within 7 working days from when you received the monitor.
There is some other conditions, you have to pay to have the monitor returned, you don't get your original postage back, and it has to be in 'resaleable' condition, so it needs to go back with full packaging intact. They won't do an exchange either, just a refund. It's also best not to mention the whole 8/20 ms thing, just say you're not happy with it, afterall it's advertised at OcUK as 20 ms. You don't need a reason though.
If you needed the 8 ms version, it would have been wiser not to get the 1905FP, since there's always the risk with it of getting the 20 ms version.
I'd advise reading up on the distance selling regulations btw.