I understand the confusion, however; it would have been easier to read if they said "Milwaukee's Best" or "PBR" of England. As far as I'm concerned, the quality of British beers is inversely related to the quality of their cuisine. If Newcastle is the shit of English beers, then I'll have to start drinking more English beers. Fuller's brewery is quite good; their ESB is easy to come by and is a fantastic summertime beer.
I second Sierra Nevada, if we're talking mainstream beers. Their winter Celebration Ale (*very* hoppy this year!) is only bested by Anchor Brewing's Christmastime Ale, where the recipe features a different theme ingredient each year (and they don't tell you what it is, making the mystery a great deal of fun).
If I had the money and only could choose one brewey to drink from, it would be Samuel Smith; their stuff is all around amazing, and consistent from bottle to bottle.
I enjoy the Guinness/Bass/Smithwick's stuff alight, but that's some pretty common beer to me anymore. In other words, I've had so bloomin' much of it that it's more drinking for drinking's sake, rather than savoring/enjoying it.
I think I had McEwan's Scottish Ale the other night. It was alright. Murphy's red is an amazingly creamy beer that I also recently tried.
Since many people are mentioning Newcastle, I recommend that you try Sam Adams Winterfest (? Winter-something, anyway). I had a sample tonight at the bar, and the color/flavor is distinctly a nut brown ale, almost undetectably similar to Newcastle. What gave it away was that it had a incredibly clear honey aftertaste (and it was a touch higher in carbonation, but that's no surprise given Newcastle's texture). It was pretty interesting, and I think many of you Newcastle fans would like it - but it wasn't what I expect when I think of Wintertime beers.