Thanks all.
You have no deadline to meet so I would send it tracked via the Post Office.
I think you mean I'm a mile ahead of the deadline, rather than there is no deadline, so just for prosperity & people finding the thread via / Google: the deadline for form 709 is April 15th the year after the gift is made. Interestingly, it's the only form I've encountered so far that also has a limit on the earliest you can file (you can't file the moment you have all the numbers, you have to wait until Jan 1st the year after the gift is made).
My concern isn't that time the snail mail will take, it's the possibility of it getting lost in the post. The late fine is 5% per month (max 25%).
[In what I see as a wonderful piece of bureaucracy, if they don't receive it on time, I can always paper-file form 8892 to request a 6 month extension :-). I don't know if there is form to request an extension of form 8892 should it also go astray though...]
I wasn't aware Royal Mail tracking worked internationally (I thought it stopped at the UK border and become ... "aspirational" after that :-)). Thanks, I'll give
Royal Mail International Tracking a try (£6.90 if <=20g, £7.75 <=100g)