You are certainly to be forgiven if you give all this talk of big GBP orders a wide berth. Yesterday there were supposedly big cable buy orders, big EUR/GBP buy orders then big EUR/GBP sell orders; it all got a bit much for poor Gerry who nearly blew a gasket.
Nevertheless, the rumour over the last week has been that there is a EUR/GBP 3 billion sell order to be executed today in London through a large British clearer. In my experience, if the market is talking about these orders a week in advance, then the order is about half the rumoured size and the bank which has the order is probably already short about half the balance.