Walnut
Unless Willie has been very active in the sale/leaseback market recently, I think you'll find your assertion re the 757s that most of the fleet is leased to be ill-founded.
As of end 2006 (excluding the props, barbie jets and 146s cut loose with BACon)
Fixed assets: 186 (incl. 13 x 757)
Leased: 57
While some of the A320 series fleet deliveries might have bumped that up a touch I doubt the up to date figure is near 50:50 yet.
Presumably you meant that BA are looking for an additional 5 a/c when you said 18 - typical that they would be looking for an aircraft which they themselves disposed of a whack of before Aviation Partners made them sexy, cost-effective and in demand again with those winglets.