Stop referring to the Allied army as 'British'. The British were a minority.
Not sure how you would define 'starting to loose'. The first grand attack failed. The attack on Hougoumont failed. The cavalry attacks failed. The attack on Papalotte failed. The attack of the old guard failed. Only La Haye Sainte fell, but only late in the battle and it gave only a minimum advantage, which was lost when the Guard (and what line troops were following the attack) retreated.
The Prussian arrival meant that Napoleon had to reposition troops to his right wing which lessened the pressure on the Allied army. That certainly helped them, maybe even saved them, but it's not like the French were winning the fight against them and suddenly had to pull back due to the Prussians.