There are are number of guides to buying binoculars on line that that certainly explain the technicalities more eloquently than I can. This is a good place to start:
http://www.betterviewdesired.com/The-Ideal-Birding-Binocular.php
A binocular, particularly roof prism designs, need various coatings to the prism an the lenses to perform optimally. Skipping the details, the difference between the budget pairs and the expensive ones there can be around 50% less light transmission. Those pairs you mentioned may seem pretty good on a bright summer's day but might quickly run out of steam in low light. Look for the terms fully multicoated and phase coated for starters. These coatings add to the price. The porro prism designs are simpler and give more bang for your buck.
Below about £300 most roofs are generally made in China. The quality is improving all the time, and indeed some are very good indeed. Unfortunately quality control isn't always what it should be so I feel it's important to either buy from a brand with good customer support or a retailer you can trust to get things put right for you if you spot a problem. Binoculars do go wrong occasionally, even the most expensive brands, and you would want to get things put right quickly. Check the warranty, but the retailer is responsible for putting things right in the first year in the UK. Unfortunately it doesn't always work as well as it should.
Depending on the brand, the list price for the entry level phase coated model is typically £130 to £200, but if you scout around for sales, ex-demos, discontinued models, used, and the like, they crop up fairly regularly close to the £100 mark. Of course you can occasionally find an excellent deal on auction sites. I recently saw £350 pairs sell for under £50, but I'm sure you understand the risks.
I'm sure many of the brands have very good customer support, but in the lower price range, Hawke and Opticron seem to get more positive comments here than some, but that shouldn't put you off other brands. Repairs might take a bit longer, but probably the retailer is the most important link. There are a number of stores that specialise in birding with rural settings for testing which have good reputations, but your local camera shop may be just as protective of their reputations and offer good service. Some of the big online retailers are fine. They might be rather anonymous but appear to readily replace problem pairs.
It's quite a while since I had a good look at the cheaper offerings I'm afraid. Just picking out a few brands that have been recommended here, Bushnell, Vortex, Nikon, Opticron, Hawke, Viking for starters. Don't ignore the more obscure names. I remember seeing good cheap pairs from Helios and Celestron which are often heavily discounted.
While a 8x42 is probably the most versatile format, you often get better deals on the 8x32s. These are often cheaper to start with and can offer wider views and better close focus. One I enjoyed trying recently was the Opticron Discovery 8x32. Yes, they won't work as well under the gloomiest conditions, but then I'm often heading home when it's that bad.
Good luck,
David
