I make the premise that I've been living outside the UK for many years but I keep some sort of eye on what's happening and I'd say that, since the 70s, the 2 (or 3) Elgars, Walton 1 and VW 2 and 4-6 can be considered standard fare. In my Edinburgh university years (1971-5) I remember hearing, in the Scottish National orchestra season, Elgar 1 twice from Gibson, Elgar 2 from Groves, VW 2 from Gibson and VW 5 from Previn. Also, Gibson gave at least one symphony per season by a living Scottish-based composer. This situation was fairly well mirrored in other British orchestra seasons and has remained stable since then.
It was not ever thus. When I was a teenager, in the 60s, we were still being told that the best Elgar was Enigma and the cello concerto, the symphonies had noble moments but were long and indigestible. Valiant efforts by Boult and Barbirolli tended to be written off as special pleading by conductors who'd been cosy with the composers.
As for the other composers you mention, the list could be greatly extended and no, they're not programmed often, if at all. Many never got beyond an initial performance, usually by the BBC, I think some later symphonies by Arnell, Wordsworth and Cooke have not been performed at all, yet the ones available in recording seem solid stuff and sometimes more. Also, the concentration on symphonies has resulted in the sidelining of composers who preferred other forms.
As for performances of these works outside the UK, reams could be written about this, but Elgar doesn't travel particularly well, whether in the USA or in European countries. When it comes to the "other" composers, it was not only a British habit, in the mid 20th century, to write more symphonies than could ever be reasonably taken into the repertoire. The USA has a similar production (Harris, Persichetti, Diamond, Piston, Schuman, Hanson ...), Sibelius and Nielsen were far from the only Scandinavians writing symphonies, lists could be produced for France, Italy, Germany, Holland, Belgium, Australia .. And everywhere the pattern is the same. Most (not all) of these works managed an initial performance. Revivals, even in their own countries, are almost unheard of.
And yet, if an orchestra set its sights on a proper examination of the 20th century symphonic production of even just its native land, it would have no time left to play Beethoven, Brahms, Tchaikovsky, Debussy, Mahler et al, and only a broadcasting orchestra in the good old days of total public subsidy could ever do that. Nor would one seriously propose such a total abandonment of the standard repertoire. It's difficult to see how performances of these "fringe" works could become more than isolated events. Perhaps we should just count ourselves lucky that a fair amount of them have been recorded so those interested can at least hear them.