SchmeckFlyer you are spot on with your deduction. It allows them to control availability of fares by market segment in the same way they control the fares themsevles.

BA in the EU may see a LON origonating longhaul as sold out in a particular calss but BA in the UK may see it has having one or more seats available, as demonstrated in the I-class example above.

I have proved to myself that they do this in the Biz and First buckets but have not yet tried comparing the WT+ or WT ones... altohough I am sure it is the same as until recently AA.com seemed to be able to book N fares which showed as unavailable in the UK.

One way of looking at it I'll put forward is this: in the UK, a highly discounted I-class fare is always FAR more expensive than an EU origonating I-fare, usually it is more akin to EU D fare. So in a sense, on a busy flight close to departure, zeroing the I-bucket in the EU but leaving it open in the UK evens out the price you are willing to sell those last seats for across all your markets while you maintain the wide fare variations futher out. Smart, isn't it?

One to Globaliser- BA is being a bit less naughty, but not yet good. Whilst I was able to take advantage of this 'loop hole' in availability and get the ex-ATH fare from the UK, they were totally unable to price a LIS fare on the same dates (and neither can expedia/travelocity etc so it must be some sort of selling restriction).