Oh... and... if you were to look at publications by Kevin Mayhew, there are several really excellent short fanfares by June Nixon - anything by her is excellent in my experience - but they are rather spread through the various volumes so it gets expensive unless you can find any second hand. I haven't the titles immediately to hand this moment, but I could look them up if you are interested. They are all about Grade VI-VII pianistically with not too much pedal and they are two sides of music - at least, the ones I'm thinking of are. Also published by Kevin Mayhew are some short fanfare like pieces by Noel Rawsthorne. And then there is this - Trinity Fanfare,composed and played by Ian Tracey, on a digital organ. One side of A4 - published Church Organ World - see here. This is very straightforward and almost as much as anything it might encourage you to write a few of your own. One day, you'll be able to write things like this, from the Cathedral of St John the Divine in New York. They have this massive en chamade State Trumpet on the west wall below a rose window. Again, a visit in person is well worth the effort. See here. They demonstrate the organ every Monday at 1.00pm and they take you into the loft.
I agree - Leighton most worthwhile, and all three of the Bliss fanfares (published by Novello) are very straightforward and effective. (Find a recording of them by Christopher Dearnley - dating from early 70s prior to the 72-77 rebuild - on itunes or whatever.) (OK - here is the first of them in which you get to hear the famous 1930 Trompette Militaire in the North East quarter gallery of the Dome. An outstandingly fabulous stop you must make it your business to hear in the flesh asap!) And the Mathias Fanfare is also very good news - published by OUP in The Oxford Book of Wedding Music - a valuable album full of useful pieces.
An album I have always found useful, published by Novello is Fanfares and Processionals. All useful and approachable pieces - especially Fanfare in D by Arthur Willis. You can find it on You tube played by Christopher Herrick on St Paul's organ in about 1968. (Here it is!) Not over difficult at all. You might get some of these second hand. Beware of shark-ish prices on Ebay.
A very reasonable source of some second hand music is a gentleman called Adrian Self who has his own music publishing company called Animus. Type Animus Music into Google and explore his site. He has a tab on his homepage for second hand music as most sensible and encouraging prices, though, sadly, not the volumes I refer to. What he HAS got and which you ought to snap up, is Four Extemporisations by Percy Whitlock, the last of which is called Fanfare. This is a big piece needing a large organ with a Tuba - I suspect you have this at your disposal! It's not as easy/straightforward as the other pieces one or two of us have recommended but you will want to play it one day. It pairs very nicely with No 3 of the 4 - Fidelis - a classic Whitlock piece of immense charm for softer stops.
My advice given a little while ago still holds - get involved with things like the Oundle courses so you can swap ideas with other young aspiring organists. Meanwhile, well done on starting some really excellent threads that have drawn a great deal of interest.