In-trim is right about BA trying to avoid duplicating the last three letters of any registration due to the ops system. It causes some interesting problems when the franchisee fleets conflict with BA's on that score.
The worst chosen registrations I ever saw on UK aircraft were two of TEA UK's 737-300s which were G-DIAR and G-NAFH. They apparently referred to the state of the engineering work to get it into service (Done In A Rush) and the speed at which Commercial wanted the aircraft into service (Not A F***ing Hope).
Previous authors are correct in that Airtours' aircraft registrations do mostly comprise directors and their wives' intials. There must be some exceptions - or else I'd be fascinated to know who 767 G-DIMB was registered after?
Other gems from the past included Manx ATP G-OATP (became universally known as Goat Pee).