are "LONDON" pressings made in the UK, inferior to the Decca pressings

Syntax,
Interesting points that in summary do not dispute or contradict the gist of what edgewear has posted.
Charles 

@syntax
"London CS was created from Decca because Decca couldn’t use their own name in USA.
(There was a small private label which was named Decca). To avoid any problems, Decca decided right from the Start to create for USA a new Name, London."

From Wikipedia, fountain of all knowledge:

Decca Records is a British record label established in 1929 by Edward Lewis. Its U.S. label was established in late 1934 by Lewis, along with American Decca's first president Jack Kapp and later American Decca president Milton Rackmil. In 1937, anticipating Nazi aggression leading to World War II, Lewis sold American Decca and the link between the UK and U.S. Decca labels was broken for several decades.[1] The British label was renowned for its development of recording methods, while the American company developed the concept of cast albums in the musical genre. Both wings are now part of the Universal Music Group, which is owned by Vivendi, a media conglomerate headquartered in Paris, France. The US Decca label was the foundation company that evolved into UMG.

@syntax I fully agree with you that the Decca engineering was the top of the foodchain back then and these recordings are rightfully considered a benchmark even today. Their consistent high quality level was not approached by their competitors, although there are titles in the Mercury, RCA and HMV / Columbia UK catalogues that are equally good, but more 'hit or miss'.

Your suggestion that the London CS were cut differently than the SXL's is an urban myth perpetuated by SXL protagonists. There's no evidence to support this. Quite the contrary, the test pressing slip is convincing proof they were created equal. Why do you insist in keeping this story alive? You don't have to worry about your investment. The snobs will make sure this remains safe.

But don't take my word (or a Decca test pressing slip) for it. The best and certainly most enjoyable (if you're unbiased) way to prove it is by comparative listening. I've compared dozens of near mint SXL's and CS's with identical matrix, mother and stamper numbers. A great example is the Albeniz Suite Espagnola (generally acknowledged as one of the prime Decca's). Comparing the SXL 6355 and CS 6581 - both with 1G matrix, first mother, B stamper (the earliest) and JT tax code - will tell you all you need to know.

Also, where do you get the idea that the London's used heavier vinyl that SXL's? Decca wasn't particularly consistent with vinyl weight, although it is true that the earliest pressings from 1958 (the so called pancakes) are much heavier. But variations in weight existed within both labels and had nothing to do with either the Decca or London label being attached.

I honestly don't think it is fair to project modern marketing tactics like 'limited editions' (or limited quantity) to Decca's way of doing business in the '50's and 60's. If they were so concerned about quality control and even melting unsold copies (?), why did they reissue older recordings in the budget SDD Ace of Diamonds series, using the very same (and by that time well used) stampers? This was common practice and Decca was a company like any other in the business of making money. They were the best in the business and  don't need these kinds of 'imaginative' fairytales to 'pique their mystique'. That they reached this high quality level using normal business practices is - if anything - an even greater achievement!

So by all means enjoy them as some of the best records ever made. If you enjoy them more with the SXL label on it, go for it. They sound great! But my intention was to give unbiased advise to (apparently) a novice collector who is interested in sound quality, not status. In the current marketplace you can buy as many as 10 London bluebacks for every SXL 2000 series with the same sound quality. Not a difficult choice if you ask me......

I've never attempted any direct comparison between Decca and London so have nothing to contribute to that discussion.

But as it happens I do have a copy of the Albeniz Suite Espanola, CS 6581, with 3G in the runout.  It's one of my favorite classical recordings, part for the music and part for the dynamic, life-like recording.  I can't imagine the Decca to be any better. 

... If you enjoy them more with the SXL label on it, go for it. They sound great! But my intention was to give unbiased advise to (apparently) a novice collector who is interested in sound quality, not status. In the current marketplace you can buy as many as 10 London bluebacks for every SXL 2000 series with the same sound quality. Not a difficult choice if you ask me......

I did not write that I enjoy SXL more, I wrote a few lines about the background in general (from my memory, it is long time ago when I collected some SXL and bought way more London CS and Monos later).
Honestly, in summary I prefer London (but can be that some titles I really like sound a bit different compared to SXL).
I was more deep in RCA Living Stereos and later to SXL and Londons...Both, SXL/London are always first rate, really bad were later the Reissues from Speakers Corner (SC=Sound Crap)

Pryso
Yes, killer definitely. I doesn’t matter what press number you have, 1, 2, 3, 4 because the quality control took proof that each stamper was first rate. You can try to find same from other cutting engineers, there are interesting differences (G, W, V, L,K, D, E ...)
G=Burkett is known for really low frequency cut.. but there is not "best" engineer, it is a bit of taste from each
A "best" sound record is not automatically 1L / 1D..maybe it is a 6W ....
I guess from CS 6581 I own 4 or 5...