I was very sad to hear of Derek Bourgeois's death. We had a very long and positive dialog since 2003. My last email to him was literally introducing a new orchestral work to him only to find out he was an hour away from death. I adore John Pickard (b. 1963) but I wonder, are there any "symphonists of note" as you describe born after 1963? :(
I don't know is the short answer to your question. There are people like Philip Spratley who has written three (of which No.3 has been recorded by Toccata) but he was born in 1942, there is Matthew Taylor (born 1964) whose three are on Dutton, there is Oliver Knussen (who, to my regret and shame, I keep on forgetting) who has written three but he was born in 1951. There are probably many others who have never been recorded or indeed performed and who work away in total or almost total obscurity, ignored by the record labels and the all-powerful BBC.
My point is that if one had drawn up a list of British Symphonists alive in, say 1970, it would have been more substantial, I suspect!
Dundonnell, you're English, right? Does the BBC still fill the same role now as it did in making these works known to the public say in 1970? I expect there are symphonies of note out there that no one knows about and unfortunately, it takes a lifetime of effort to master the form the issue is by nature of looking for "symphonists of note" you'll be looking for someone who has a lot of life behind them.