You just need to press the landing gear override button, and then move the gear handle up. If there is hydraulic pressure, the gear doors will open, and it will attempt to retract. The main gear almost certainly won’t move, as it is dragging the gear at 90º to the tyres (and because there’s normally some form of positioning required for the gear truck). The nose gear doesn’t have anything restraining it, as the retraction is in the same plane as the tyre rotation. There are maintenance procedures that require this but the gear pins should be in place to stop any result, other than the doors opening. So, either the NG pin wasn’t installed, or someone had put it in the wrong hole (which apparently was also done by BA).