In general, your friend is right. But that typically applies to mass-produced PCs from the likes of Dell, HP, Acer, Gateway, and so on. Plenty of boutiques and PC shops around the world build solid, affordable systems in-house from the same parts DIY builders tend to use. So, not all pre-built machines are total ripoffs. Sure, they charge a fee to assemble them, but people who don't have the knowledge or time to build their own can still get a pretty fair deal on such machines.
I'm in the US, so I'm not familiar with PC component pricing across the pond. But, here are several UK PC component (and system) vendors to consider:
One thing you may also want to include is the resolution of your monitor, as it's a contributing factor towards GPU selection/recommendations. I could hit those sites and offer suggestions, but it's rather troublesome when I don't comparison shop UK vendors enough.
The GTX 570 is a good graphics card, and couple that with an i5 and you have a good setup. You should do some research yourself (get an idea of how everything works, terminology, models, a build of your own, etc) and come back with more specific questions.
A Gen2 Core i5 2500K (the most value-packed model) + GTX570 system would be near impossible on a 600 quid budget, though. Peeking around Scan's site a bit, the 2500K is 170, decent P67 boards start at about 90-100, and GTX570's start at 252. That only leaves about 80 quid for a case, RAM, and a HDD. It's just not possible. You could save a few quid here and there by choosing an i5-2400 paired with an H61/H67 board, then going with a GTX560/560Ti, but you miss out on a lot of CPU potential (via overclocking the 2500K) for very little CPU cost difference. Most any GPU from the 460/560/6870 range and up is going to be pretty damn God-like at 1440x900.