Pros and cons of cycling to work and back 5 days a week?

Balls. It's better than fuck all exercise (sitting on the tube) and if you push yourself it's a good 50mins-1hr of exercise everyday.
How can that not make a difference?

Yeah, stop riding for a month and then start again and see if you don't notice it being harder than it was. My money is on the fact it will be.

Also, you probably just eat more now, overcompensating for the 14mi/day, that's why you weigh the same. Another option and this is what happens to me is that your weight is very similar but you are carrying less body fat and more muscle. Weight means fuck all if you don't take into account body composition, body fat, etc. When trained, I look a lot leaner (relative you fucks!) than when I'm slacking off (like the last 3 months) and yet my weight only varies by a couple of kilo (fuck all when you're 95kg).