Odd Skipper..............The deal is that if BA hire you it is a 5 year engagement freeze on type. However, if it suits them they can waive this freeze if they need a fleet manning and there are insufficient volunteers or there are insufficient qualified pilots.
An example could be LGW shorthaul which needs Captains but not enough people volunteer who have the licence, hours and experience requirements. If you were to volunteer, through the normal bidding process, and were found suitable then BA may well offer you the post. All done in seniority order........