I managed to lose 1 stone in 3 month, but my work out was rather not intensiv. 2 stones per 3 month is possible, but you need a very high activity and a proper meal plan.
Your higher weight means that activity can cause ligament or joint damages. I would recommend you to get 100 % collagen supplements. It will improve your protein intake too.
Swimming is the best sport for a quick weight loss, because it has very low chance of injuries. Just avoid cramps. Combine it with high intensity interval training to maximize energy output.
Keto diet is the fastest way to lose fat, but it is not easy to maintain. Personally I have switched to low GI diet, which is not so effective, but better for beginners.
I would recommend you Athlean X on youtube for very hard gym workout sets. He doesn't prefer keto diet tho.
Start can be pretty fast and you can lose 5 kilos in a week, but further it become more and more difficult. Let us know how you are doing. I wish you good luck.