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

I was taking an electrical engineering degree at Imperial College in London during the early 1970's and I had the opportunity to work for Decca at their New Malden plant during one summer vacation, basically acting as a gopher in the pressing plant.
I can state categorically that the same stampers were used for both the Decca and London pressings at that time and the requests for metalwork often included both the US and UK labels in the one invoice- which explains why both issues often have the same stamper codes in the inner section.
I know of at least one copy of such an invoice which definitively proves that there is no difference.

Having said that, I have numerous copies of both SXL and CS issues of the same work and more often than not the SXL issues- most of which I acquired in the UK- sound better than their UK pressed CS counterparts- most of which I acquired in the US. The reason for this is entirely unclear, except that my SXLs were bought new, while the CSs were used.
US pressings of CS issues are generally inferior and are not worth having.
Phillips pressings can be quite excellent.

@wynpalmer4, thank you so much for sharing this information. It really is most valuable to get first hand confirmation that UK pressed SXL and CS labels were created equal by someone who was actually there!

But in your experience the SXL's are still often sonically superior. I'm sure this will delight the SXL fundamentalists, even if the reasons are unclear. In your case 'new versus used' might go along way to explain this. Another possible explanation could be that with each pressing run the first part was given the SXL label, while the latter got the CS label. In that case the slightly more used stampers for the CS labels might explain a slight sonic difference in favor of the SXL's. But this is pure speculation on my part and it's probably not very likely that Decca would stop halfway a pressing run to change labels. Having been there, can you shed any light on this? 

I've never been able to detect any generic difference between UK pressed SXL and CS labels with the same lacquers and stamper codes. In some cases the SXL sounded better, in others the CS. Because all my copies are previously owned, I tend to attribute these small sonic differences to their past playing history. This will always remain 'the great unknown' in collecting used records.

i cannot hear difference

First off one big difference is a lot of changers in use in US, not the case in UK. Also many times the Decca LP was put out first, so more likely to have fresher stampers. Charm has all the details on which LP came out first for most titles:
http://www.charm.rhul.ac.uk/discography/decca.html

This is the main document at Charm:
http://images.cch.kcl.ac.uk/charm/liv/pubs/DeccaComplete.pdf

Here is a sample entry:
>0974
Pr: James Walker (m) Eng: Kenneth Wilkinson (m)
Pr: Erik Smith (s) Eng: Cyril Windebank (s)
15-16 Jan 1957 Kingsway Hall
Ruggiero Ricci (violin), London Symphony Orchestra, Pierino Gamba
BRUCH Violin Concerto No.1 in G minor Op.26
MENDELSSOHN Violin Concerto in E minor Op.64
(Jly57) LXT5334 = (Aug58) SXL2006;
(Sep57) LL1684 = (Sep58) CS6010, (May89) 417 793.2DM.

Here we see SXL2006 in August of 1958 versus CS6010 in September; advantage to the SXL.

Works like a charm for collecting Decca/London!

I'd lean towards SXLs because the LP was more likely played by a good turntable and needle. In the US YMMV with lots of people having nasty changers and needles plus poor LP hygiene habits. Very pleased with British dealers and the quality of their LPs.

I have thousands of records many London i have always found the Decca to have better sound even though they were pressed in the same plant.Go figure.