Well, firstly it's £305m, which translates into almost $600m at current exchange rates. The price of a 737-800 is $65-75m (similar for an A320 I would imagine, although I can't find the prices at the moment). Let's say $70m for arguments sake, list price. Big airlines like BA can get good discounts on this price - I've heard up to 40%, but it's not a mega-order (like a Southwest or Ryanair deal) so let's say they'd get only 30% off the list price, that makes $49m per plane.

So roughly they might be looking at 12-13 737 replacements with this type of capital expenditure? Of course I could be way off. Obviously not enough to replace the entire fleet, but there are maybe a dozen or so 737's that were built in 1989/1990 so maybe these will be the first to go?

Interesting most of the 737-400's are younger than they look (you wouldn't think so if you'd travelled on them). Most delivered in 1992/3/4. That makes them only 13-15 years old at present, which is well within design lifetime and will be for some few years yet.