You have to do any processing? I just hit the "download gpx" button on strava, and it gets me a GPX, I sent this to the DIY Org.
I would call that a processed file. Strava very possibly smooths the data, re-writes the elevations, etc etc.
But I do understand that for many people this is the only practical workflow now.
The validation tool accepts .fit, so I suspect the FAQs for GPS may be a bit out of date. Or maybe it depends on the organiser. I keep meaning to ask Martin about it on one of his rides, but I usually forget because I'm busy having fun or stuffing my face.
The thing about GPX is that it is an open, cross-platform, documented standard. Other file formats are not. FIT is very good (a big improvement on GPX in every way EXCEPT cross-compatibility) but it has evolved (and who knows, may evolve further) so that not all files with the extension .fit are equal. This requires whoever is dealing with these files to have modern software kept well up to date, simply to be sure of being able to access the contained data.
To be picky, the AUK validation tool does not itself accept FIT (the software has not been updated in a very long time) but the file uploader does accept FIT and converts it to TCX which is a format the validation tool understands. Unfortunately this conversion only works on older, un-evolved FIT files, and fails on files generated by newer Wahoo devices for example. There is a note on the tracklog uploader form, that "Wahoo .FIT files may not convert correctly".
But I think this is by the way because, for various reasons, most organisers don't use the validation tool anyway. Which again means that the whole process is compromised - because the tool does what it does very well (even if it could be a whole lot better), and I think it's highly unlikely that any organiser simply viewing a submitted tracklog on a map for example, will perform many of the essential background checks that the validation tool does.
Interesting reading that information in the FAQ - it contradicts itself really - it asks for a 'raw' file but then encourages topping and tailing, and joining files - all of which I think should be better avoided (though I'm not a DIY organiser).