Best to order halal as special meal as that certainty you are looking for is not possible. BA changed its suppliers years ago and since, even internally it's hard to get a straight answer from them. I work there and have tried many times. Upgrading at airport using 'airport upgrade promotion' is possible if you have a special meal request. When the AUP's were first introduced several years ago, it was not possible. They will now serve you the original class meal in the upgraded cabin. Operational upgrades (free surprise ones) usually won't happen for special meal customers, and now there's an internal robotic using algorithms which determines/processes the free upgrades. Brought in to eliminate possible corruption. Top frequent travellers, high yield tickets, service recovery for previous service failure etc are all part of that.
Food is picked up at outstation for all the return segments of flights to KSA, UAE, Pakistan etc (ie longhaul) so return meals are halal. Not picked up for mid haul (ALG, CAI, RAK etc) so not guaranteed halal. Hope this helps.