Is it worth building a custom PC vs buying a prebuilt one.
Yes (with the exception of a $400 or less computer), this is because you can get at least comparable parts (but normally better) for the same price. This also gives you parts that allow you to upgrade in the future.
As for PC gaming vs counsel gaming I believe in the long run the PC gaming is cheaper. This is because you don't have to pay subscriptions (xbox live) and the games are much cheaper. Although the up front cost is much higher by about three years they've evened out or are about the same if you get the same number of games every year.
Lastly if you don't feel like spending 700 pounds you don't have to. You can build a good gaming build for a couple hundred cheaper. On a counsel BF3 runs at essentially the lowest settings so even if you buy a $120 GPU you'r better off, so you don't really have to get a $450 GPU to be happy.
Think of it this way if you keep your computer for gaming without any upgrades for about 4 years and have to run games on low settings (which I doubt then because the lowest need to be able to run on a counsel). So probably medium setting you're better off than a counsel which at the moment is running games like BF3 at their low settings.
Definitely build. You can get prebuilts, a lot of those seem to not last much at all. My custom is about 5 years old. About the only thing I've not upgraded is the motherboad. My PC started life as a Windows Vista box with an Athlon dual core and 2gb of ram and a 7300LE video card.
Since then, I've upgraded to a quad core processor, 6gb of ram, SSD along with a 1TB drive, new case, newer power supply, and a GTS 450. It now runs Windows 7. For now it does ok, but this year hopefully it will get a new power supply, maybe a Radeon 7850 or 7870, but I also like the GTX 660 cards as well, this month I'm probably going to drop Windows 8 on it. Keep in mind, my pc is 5 years old, and I still have a 7.0 on the Windows Experience Index, where I'm limited at is my board I think because I don't think it can use the full potential of my SSD, but I figure for gaming, I'll get more bang for the buck with power supply and graphics(want to get a 27 inch monitor one of these days also).
Point is though, my pc is 5 years old, drop almost anything out there in it, and it runs. Most systems from big box stores people are nearly ready to throw them out. Mine is still going strong. You get more options in my opinion with customs. And too, they last longer it seems. That side, vs I've worked on at least a few prebuilt machines dead in only a year or two. Saw an acer once with a bad motherboard at 4 months old.