Conspiracies, conspiracies.
The new A321 delivery is part of the BMed/GB joint order under BA buying power, so BA is the initial acquiror of the aircraft.
On the CAA website, an aircraft is shown as "chartered" if it is leased as opposed to owned outright by the airline. If, for example, you look at Thomsonfly 757 G-BYAF, it shows up as "chartered" because it is actually owned by leasing company AWAS. It doesn't say anything about an impending merger of Thomsonfly with First Choice.
GB's fleet is mostly owned by leasing companies, hence why they come up as "chartered" on G-INFO:
Royal Bank of Scotland aviation finance own four of the A320s
Aviation Capital Group own three of the A320s
GB owns or has finance/mortgage leases on most of the A321s
From what I understand, BA have only allocated replacements for the leased 737-500/300s in the fleet of which there are coincidentally 14 aircraft. The BA-owned 737-436s (of which there are 19) will remain in service for some time. 19 + 14 = 33, which is the current number of short-haul aircraft that BA operate from Gatwick.