All these bikes are substantially similar, kit- and frame-wise. Starting from the lowest price and going up, you'll notice fairly dramatic improvements for small increments of cash; the Cannondale Quick series is a good example. The cost difference between models from the 8 to 7 to 6 etc. are below $100, and the ride quality changes measurably. One money-saving effort is to look at a previous model year. The prices are usually discounted, and last year's bikes were pretty much the same as this year's.
I'd suggest you try out the low end of your price range, and the next one up, and see if it's worth the difference to you; don't try out the one 'just above' your price range, because that way lies madness. The ride you can afford is perfectly adequate these days - the cheapest Shimano group will shift and brake just fine.
What will matter among brands is the fit. If you sit comfortably aboard the Malvern Star Sprint, you'll be much happier over time than if you choose a less comfortable, say, Giant, with better componentry.
They're all reticent to tell you the weight, because they're all heavy. There may be an alternative for you; if your commute is mostly flat, it might be worth considering a single-speed bike, like the Raleigh Back Alley, which will cost about the same for a substantially lighter bike, if you can manage with a single gear.
Hope that's some use. If you'd like specific model recommendations, I like the Cannondale Quick 6 or above if it's not outside the budget, the Merida for the middle range, and the Trek is the best of the cheapest.