John,
Personally, I'd do the following:
Buy an L ticket (approx. £450), and upgrade it to PE (20k miles) now.
Between now and then, and I mean right up until the day before, keep an eye on Upper Availability for that flight, just in case it opens up.
If it does open up, call FC and "pay" the extra 40k miles to upgrade to J.
If it remains closed, you have an upgradeable ticket type to try your luck at the airport.
This way, at least you're on the flight. Worst option is that you'll be flying PE. Best case, availability opens pre-departure and you can upgrade beforehand to J. 2nd best would be an upgrade on the day, at the airport, on your upgradeable ticket type. [y]
(PS - you'll also earn 13k miles on the revenue element of your "Reward" ticket as an Ag holder...

)