At the moment Avman, Apron Control (BIA ltd) allocate Stands for aircraft. Apron Control is that tower looking thing between the old international pier and new one adjacent Stand 55. This will change as and when the new "Project Blue" centralised Control Room comes on line.
There are quite a few ATM's at BHX now with limited stands due to WIP. You were lucky to get a terminal stand twice!!!
Stand allocation is programmed into the "FIDS" system the night before the following day operations begin, both T1 and T2. Unfortunately, aircraft go tech dumping Jet A1 or hydraulic fluid, aircraft come early, go late. Stands / Airbridges go tech, Fire Alarms actuate. This will then affect this prior planned stand allocation programme. Apron Controllers will then have to look for alternative stands depending on what they have at the time.
Hope that answers your question.