I understood you had to register well in advance,before you could even file a plan.Did all you guys do that?
As far as I know this only applies if you use SkyDemon Light. Which is the free flight planning tool on the internet. Tim Dawson has extended this with flight plan submission functionality specifically for the Olympics. As you don't need a SkyDemon account to use SkyDemon Light, he had to use a different method for validating you're a genuine pilot and not a spammer. That validation is done through the CAA and that takes about a two-week turnaround.
If you register for the full SkyDemon package, including a 10-pound deposit for flight plan submission credits, you can start using SkyDemon to send FPLs within an hour. (And plans submitted because of the Olympics are free, so will not use up any of your credits.)
You can also submit through AFPEx, which is the other option for UK-based pilots to submit flight plans in general, including FPLs for international flight that have nothing to do with R112. However, AFPEx apparently requires you to format your FPL message by hand and doesn't do a lot of syntax checking. This is the place where most people get it wrong (like forgetting EETs to the FIR boundary, which SkyDemon does automatically) and where most of the rejections seem to come from.
And of course you can submit flight plans through any other FPL interface. I have seen RocketRoute doing some advertisements, and I personally plan to use homebriefing.nl if the occasion arises.
So anything you are currently using to submit FPLs (e.g. for international flights) should work for Atlas too. But do make sure the message is formatted properly.
(And if you get a rejection from Atlas, it still falls on you to send the cancellation message to all recipients of the original FPL - they won't know of the Atlas rejection. And if you submit a new flight plan, use a different EOB time so that it clearly is a new FPL.)