Along with the late 1970s Transavia 737s, BA also leased in Britannia 737s at the same time as well, an unusual sight on the jetways at Terminal 1 (in fact I can recall side-by-side with Transavia). There were multiple issues with the Trident fleet over time which required significant mods and the fleet available being reduced. Other "borrowings" were surplus Super VC-10s from the long haul side, sufficiently permanent to appear in the timetable as such, and of all things an American Trans Air 727 which remained on the US register (and with an ATA crew) but with minimalistic BA titles applied. This normally served as one of the backups on the Glasgow Shuttle, presumably it had been leased by the hour.
The Airtours fleet were also commonly over at Heathrow in the off-season as substitutes, as they had been ever since initial Comet days.
1 year rolling leases that go on are not as uncommon as one might think, the first American Airlines MD-80s in the 1980s were done on the same basis, known as "Sweetheart leases", because it's easy to say "Sweetheart, goodbye" (apparently !), and those not only ran for the life of the aircraft but were precursors to a fleet of about 300. The financials and terms are generally renegotiated as time goes on.
If you want some good detail these downloadable spreadsheet-format lists seem to be as quickly searchable and as accurate as you can now get, including the longer term leasings
http://www.airlinerlist.com/