BMI is a very, very indicative thing. As in, it's not a precise measurement at all. Depending on build, muscle and some other factors, someone might seem like they're absolutely lean and fit at a BMI of 25, and their neighbour will look like a fattie.
Basically, I think that by "BMI of 25" they mean "healthy weight". If they see you have a healthy weight, it should be all right. If you're really scared, you could try to have someone (like your GP) measure your body fat percentage and write a letter certifying your body fat percentage is X and that it's a healthy value, definitely low enough to be safe for surgery.
I doubt any doctor bases themselves off a volatile thing like BMI in such a strict way though, so such things shouldn't be necessary. You can write the surgeon if you want to make sure, too.
I, for example, am still a little fat yet my BMI is around 22.5. But my mother, who's technically overweight, says she became literally skeletic and sickly when she technically reached the middle of the healthy weight range. And my sister, even at 170 pounds (well I don't see her often and her weight yoyos a lot), doesn't look THAT fat. And on the other end of the range, some people are, like, underfed according to BMI, but look and feel great. (I envy those people, but this is not the point.)