Pros
The London based Dev and QA teams are very good; interesting technologies in use (eg. Solr, Druid, various JMX), proper range of data centres including (at least) dev, user-test, load-test, staging and prod environments, all physically separate.
The SDLC architecture is very good too; Git, Maven and Bamboo, and an impressive version managed deployment automation system built in python.
Social feel of the place appeared OK; I'm not an expert. Salary was competitive.
Cons
The technical architecture comes from a sister firm, out in Malaga, Spain. You may enjoy this; I found it to be a PITA.
All the most interesting software development is likewise not originated in London; it is essentially vendor supplied, although pretty tightly integrated and closely supported.
Management in the London office is complex, confused and overlapping.
Operational procedures are weak. The guys in the Spanish office have a tendency to point and laugh rather than helping,