On 737s....
Airfleets gives 14 aircraft with WW at an average of 15.8 years.
BA is given as 19 aircraft at 19.6 years average.
The BA B737-436s were bespoke ordered and paid for. They are surely paid off, whereas the WW fleets is leased. So they are (re)introducing an old subtype with fewer seats that is going to need a fair amount of cabin work to come up to BA standards, unless they are going to keep WW seating and interiors.
I believe the plan for LGW has not gone to the board so there is no scope to order a replacement for the B737 until then.