There are certainly differences between brands of extracts. Maltsters tend to use ingredients local to them. For instance I'd expect Munton's to use English malts, Weyermann to use German malts, Briess to use American malts, etc. They defiantly rely on local water supplies which may be drastically different from country to country, region to region. Water will definitely affect their finished products. Maltsters also likely use different processes for preparing their wort and reducing it into extract. They also likely use different recipes for some types of LME that rely on a variety of grains, like amber or dark LME. All of these factors can impact your beer's flavor by providing differing mineral content, fermentability, and base flavor.
Can I substitute Northern or Briess Pilsner LME for Munton's or Alexander's? Thanks.
I think it depends on what you want. If you are trying to clone a specific English brew, or if you are trying to brew something that strictly adheres to a specific English style, you'll probably get closer to your goal by using Munton's. If you're just interested in making a delicious beer based on an English recipe, save some cash and use the NB or Briess.
Extract freshness is ultimately more important than brand. If you get stuck without fresh LME from a reliable place like Midwest or NB, it might be better to buy a product in a can (or DME). I think cans provide longer shelf life than some of the other extract containers. Cans keep light out. The top doesn't need to be taped to keep it closed. They're generally more durable. So if the choice is a can of Alexander's or filling my own bucket out of big plastic barrel of Briess that's been sitting in my LHBS for as long as I can remember, I pay the extra cash for the can.
Anybody know who supplies the LME for NB??