Alistair, some help here, I know this is getting in the mud with the swine, but anyway.
Elgar, Holst, Vaughn Williams, Bax, Britten, Sorabji, Tippett. And the way you put it you would also capture Purcell, Byrd, Dowland, Bull, Gibbons.
S'il vous plait.
I cannot help you here, because the premises are too flawed. Of course there was a period in which the accusation of the "land without music", whilst something of an exaggeration, had sufficient credibility to hold water during the period between the death of Purcell and the rise of Elgar, but British music had so much going for it outside that fallow period that "crippledsymmetry"'s comment on this can simply not be taken at face value and believed. The rest of what he/she writes is far more credible, however...
There isn't necessarily any association with some 'old' complexity intended so much as a distinction from the simplifying tendencies of music that were in the air during the 70s/80s. I suppose that the most obvious precursor is the post-war avant-garde, especially the serialist and stochastic techniques developed then which have been most influential on the composers in question, but given most of their other musical interests one could just as well point to the English madrigalists & ars subtilior...
As far as I can tell the 'new complexity' branding was invented (not by the composers) in order to promote the music on the continent, at a time when British music was not really taken very seriously due to being crap for centuries. Most of the composers concerned dislike being grouped together too strongly as they all have highly individual approaches to writing music and a general distrust of categorising, branding etc., but the label was probably useful in establishing them all as a real breath of fresh air in the British music scene and important presence in Europe generally.
It's certainly true that the composers didn't invent, or see any kind of PR advantage in, the term "New Complexity" - which is just as well, really, since - as Sorabji observed in the 30s - what might be promoted as so very much "in the mode" and the latest fashion in "musical haberdashery" was - and is - likely to become yesterday's news all too soon, so any such term that seeks to bind together some group of people under the name "New..." anything at all carries with it its own inherent risks.
Britain certainly suffered from a backward-looking stance in the mid-20th century which saw a strong resistance to having anything to do with certain continental persuasions, preferring what it perceived as some kind of "English" pastoral realism, yet composers such as Searle and Lutyens became first branded as modernist enemies and then old-hat serialists in a period which saw the coming and going of William Glock's somewhat despotic (though at least in part necessary) régime at the BBC following Searle's input there and before the débâcle with Simpson. The whole thing can now be seen as faintly nonsensical, since we now accept George Lloyd and Brian Ferneyhough as British composers who at a certain time worked concurrently to produce music that might to some seem as though it originated from different planets but which was still the product of British minds (OK, Cornish in the case of Lloyd).
Nowadays, it's a free-for-all everywhere to the extent that "anything goes" (at least up to a stylistic point) and the remarks made by Schönberg in his book
Style and Idea when defending and explaining his continuing need to write tonal music after having developed his "system of composing with twelve tones related equally only to one another" and claimed that it would "ensure the supremacy of German music for 100 years" (which Ronald Stevenson once wryly remarked was a "strange idea for an Austrian Jew to have") were not only pertinent but percipient and arguably also strangely prescient of the present age.
Reading a statement that "British music was crap for centuries" would first prompt me to ask whoever had the stupidity to make it (a) which particular centuries he/she had in mind and (b) what it was that supposedly singled out Britain for this gross accusation (notwithstanding the reservation to which I drew attention earlier).
I'm not sure that this has helped, but I hope it has.
I would, incidentally, lay a minimum of a thousand pounds to a penny that the true identity of "crippledsymmetry" is not Colin Matthews...
Best,
Alistair