I'd recommend saving for a bit longer to be honest.
The games you've mentioned aren't the most graphic-intensive, although Portal 2 probably needs the highest spec machine, so look at the minimum specs for the game and see what sort of PC you should aim for.
Remember that PC hardware moves along all the time, and the games do to, so what suits him now might not be up to scratch in a year's time, say. Although at least you can put in a better graphics card for not too much money at that point.
On-board graphics cards are pretty rubbish, and no "gaming machine" would ever use one, although you can pick up a fairly decent graphics card for not too much money these days. Certainly a card that'll play the games you listed anyway.
I think your best bet is to go for a pre-built machine like the ones you're looking at, and think about getting one with a one or two sub-standard bits in them (usually it's the graphics card that's not up to scratch) and then buying a slightly better graphics card and putting it in. It's often cheaper than buying a machine that's got all the fancy bits in right from the start. But for £275, you probably won't get too much.
If Sammo were here, he'd recommend looking at
Ginger6 for some cheaper PCs. I'd recommend looking at
Big Pockets. I've bought a system from them similar to what you're after and they seemed pretty good for the money. I bought the machine, and a new graphics card, and I had a machine that lasted for a long time. They also do refurbished machines too, for a little less money.
