Skylon has an advanced propulsion system (or will have) but the SR-72 has a totally different purpose, and needs to be able to deliver weapons on target. Certainly there are already planes that have gone faster than the SR-72 might, but none in the same job. I understand the reasons why the want this plane, but I still feel we have the assets to do the job now(minus the attack profile it would have). We are going to have to look at reduciung the over-inflated defense budget at some point...most likely before this plane would ever enter service.
Obviously the plane doesnt exist yet, however Lockheed has a good record at getting these projects off the ground and funded.
This aircraft would have been pitched secretly to the Air Force and had it been accepted, we would not be hearing about it now. It must cost too much and they are trying to get members of Congress to fund it.