I was there a few weeks ago.
If you're staying in a civvy hotel, you're entitled to $69pd actuals. If you're on-base, in an Air Forces Inn, for example, you're entitled to $30pd actuals. Upon return you're also entitled to £10 MIE rates for each 24-hour period you were overseas. Remember to keep the bl00dy receipts! Someone told me, though, that you can claim on actuals for amounts less than $5 without a receipt; I'm not sure how accurate this is.
If you're applying for an advance on JPA you need to put in the claim dated the day you submit it, NOT the date you require the allowances. Despite JPA's many problems, I got my advance 5 days after submitting the claim. I'm not at work yet, but when I am I'll post details on how to claim a foreign currency advance, because if you miss one tiny thing you'll not get the money. Flashback to my boss bankrolling an SAC who couldn't get his advances because HR gave him the wrong information...