Makiko is right that the allowances are very generous and you’d almost never spend them all whilst away. They got much more generous in the last EA as you get them even while flying, not just in port.
However, his quoted figures are grossly inflated.
$400 per day is NOT the average, it is the absolute Maximum you get anywhere. Only get that in SIN and Korea as best I can tell, not LHR.
A 9 day LHR does not get 9 X the daily allowance, it is based on hours away. It’s 183 hours which is closer to 7.5 days and nearly half the time you’re on the lower UK rate (band 5).
As mentioned above, if flying to BKK, MNL, SCL, JNB, CGK, DEL, BLR…. Then you’re getting less than $300 per 24 hrs.
The system has only been in place less than a year so no one can tell you exactly what you’ll get but I’d guess 30-35K is a good estimate.