I'm not at all an expert on this matter, so take my comments below "with a grain of salt".
1) Unless you have posted a bond or otherwise had to make some kind of contingent liability with the EPA to get the permit, I would not be too worried about closing it out. You don't reside in the US, and it is unlikely they will try to come and get you in the UK as if you were Julian Assange or similar.
2) On your way down to Ushuaia you will probably collect dozens of documents recording importation & exportation of your motorcycle into all the different countries that you pass through. I think it would probably be sufficient to forward to the EPA the documentation showing that you exited the USA into Mexico, and then some additional documentation that proves further southward movement of your motorcycle after leaving Mexico.
3) If all else fails, when you return to the UK, you will have lots of documentation of that (your shipping waybills, etc.) - just send that documentation to the EPA, it will show that your moto is back in its country of origin.
Hope these thoughts help.
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