If the Infinity Primus P362 goes on sale at Fry's again for $99 a piece, you can get 7pcs for a 7.0 system for $693 + tax. Yes, you can use the floorstander as a center _IF_ you have your TV wall mounted/elevated. Seeing as you have a 6m x 5m floor area for your garage home theater, I don't think space would be an issue for these speakers.
EDIT: As I was writing the first paragraph, I forgot that you stated you live in Australia so it's probably almost impossible to get the Infinity P362 for the $99 sale price.
For JBL I heard some of the L Series, namely the LC2 and L880 (Or you could get the L890s if you want more bass extension due to the 8" woofers compared to the 6" woofers of the L880). They sounded sort of similar to the Infinitys but I found the highs a bit on the harsh side (This is subjective, you might find them to your liking so it's really best to audition speakers before buying), I figure that might be due to dual tweeter design (They have an additional ultra-high frequency tweeter in additional to the normal tweeter).
For UK brands, the only brand I know of is Wharfedale (Diamond 9 series). I frankly didn't like them because they sounded too "laid back."
For your subwoofer, I don't know what brands you can get in Australia. If you live in the US, people would suggest brands such as Hsu, SVS, Elemental Design.
Matching the brand of the subwoofer is not important. But matching the 3 front speakers is important. Surrounds can be a different brand from the fronts IF you're going to be mainly using the system for HT. If you're going to listen to music and have all speakers replicate the stereo sound, it will come out unbalanced.