I have the tarty 1.25 tonne ali Machine Mart jack
http://www.machinemart.co.uk/shop/product/details/ctj1250a-11-4-tonne-aluminium-trolley-jack and it still lifts a corner of the Omega quite happily. Struggles a little bit but mainly due to lack of mechanical advantage as it seems "higher geared" than some of the heavier duty ones.
I bought it primarily to fit under the Westfield as it goes down seriously low. It also spreads the weight at the front across a big roller rather than 2 narrow wheels. Still doesn't roll that smoothly (on a smooth concrete floor) when there's load on it though. You can help the issue by making sure the wheels / rollers are well lubricated. They often aren't well oiled during manufacture and only get worse after languishing in a garage for ages.
I think you're always going to struggle on tarmac unless you spread the load somehow. A bog standard old scissor jack might even be a better bet on this type of surface. It goes down low enough, and lifts straight up, with the added bonus that its' smaller footprint on the ground means it leaves more room to get an axle stand in.
I remember once I had to change a wheel on a glider trailer on a fairly soft airfield. By the time I'd finished I had to dig my trolley jack out. It was completely embedded in the ground.
Kevin