Registration Selection
When I worked for them, the BA Engineering Quality department would research availability and agree suitable registrations, based on a set of guidelines,
with the delivery project manager, who would then circulate the suggested registrations to interested departments for comment.
If there were no objections we reserved the suggested registrations with the CAA.
Sometimes specific letters were requested, such as G-EU for the A319 and more recently. G-GAT for the LGW based A320 aircraft.
So in answer to your question, the initial suggestion could often originate from relatively low down the food chain, but the ultimate
decision was effectively taken collectively in conjunction with other interested departments - not sure what happens in other organisations.