Honestly, aiming at 50Â£/week at the start without having any prior experience or a "plan" that would make it work, is kind of unrealistic. I haven't ever done it, but I think I've been around long enough to have an understanding on how it works, anyways a good start would be getting reputation on multiple forums. If you want to sell gold for more than you've bought it you have to buy/sell it at different places, no one is going to give you 1.3 in a forum where somebody else is selling for 1.2.
You also have to keep in mind that you're going to need a lot of cash to invest, and it may take a while for it to turn profitable. If you want to make 50Â£ you will probably need like at least x10 that value in order to buy/sell the gold, and be ready to hold a lot of gold, because it's easier to buy than it is to sell. Mostly at the start, you'll have to be ready to barely get any profit at the beginning in order to get a decent amount of attention/reputation. Beginning is tough, but if you find success it can become really profitable with very low effort.
One good way to get cheap gold is to buy it directly from new goldfarmers and whatnot, you will also have to sell it cheaper than the big dogs(very popular sellers, websites and whatnot) also be ready to be available 24/7, when most people want to buy/sell gold they want to do it ASAP, especially the "low-level" farmers who need the return of the gold to set up their farms, or don't want to keep their gold for too long on mules and don't feel like selling their gold for half its value to the big bois.
I think the key to reliably getting cheap gold is being available, and getting as many contacts as you can, as far as it goes for selling it, earn trust and be able to be reached as often as possible. So yeah, it ain't that simple. But that's just my 2 cents.