My thoughts are as follows..
As a rule, if your training, you may be losing fat and gaining muscle. This is why measurements and photos are a better way or gauging fat loss ( you say your friend think you've lost weight ), unless you can get an accurate body fat analysis done.
If you have a calorie deficit, you'll lose weight/fat. At 15 stone, and training most days while consuming 2100 cals, you'll definitely have a deficit.
I find cycling is best for losing weight. Steady Zone 1, possibly fasted, for a couple of hours.
I went from 16 stone to 12 Stone over 12 months, this is what I did: Train without gels and just add water to your bottles. Reduce the number of beers just one or 2 at the weekend, dramatically reduce the amount of carbs in your diet - meat, cheese, milk, butter are all fine, reduce bread, rice, pasta - I was amazed how fast I lost weight doing this
Myfitnesspal allows you to eat back your exercise calories, so you may as well not burn them at all!
I would set your activity level at zero and then don't eat back your exercise calories. This is what I did and it works for me. I also find that myfitnesspal overestimates the calories you need so google a TDEE calculator (scoobys or IIFYM are good) and use those instead and set your own goal in MFP.
As above the scale does not distinguish between muscle and fat so try to go by how your clothes fit and take progress pics/measurements instead.
Nutrient timing - in this country we are very bad at this and often back load our food, i.e. eat our biggest meal at dinner time. This means your body is in energy preservation mode for most of the day (as you haven't given it enough food) and then dump a load of calories in at dinner time. Your body thinks "good, I've been fed, as we've got some down time (sleep) lets stash this away to cling on to it for tomorrow" this means you can be theoretically in a caloric defecit but because you put your body into starvation mode during the day it isn't burning as many calories (so its not as much of a deficit) and then you give it plenty to store up in fat at bed time.
It's far better to front load your calories consumption and try to avoid eating too much within a couple of hours of bed time, that way your body isn't able to pack on fat whilst you sleep.