Eagleray -- you say you have been in the UK for nine months, right? So, you're still legally permitted to drive on your US license. But bear in mind that, as meggles states, you'll only be able to do that for another three months. You may want to think about biting the bullet and taking your test to get a UK license. Or ... and this is entirely up to you ... risking it and driving illegally on your US license. I wouldn't recommend that, but it has been done.
When you get stopped by a speed camera, the camera detects the car. The car is obviously registered to your husband, which is why he got the letter. As Meggles said, he'll need to fill in the form and post it back stating (in the space provided) that you were the driver. You'll then get a form of your own in the post. At that time, you'll need to let them know that you're driving legally on a US license.
If you later decide to get a UK license, then the points you were given for this infraction will transfer over automatically to your new UK license. You'll have to be very careful from then on out (for your final year in the UK) because I believe that as a new driver you can only get two infractions before losing your license.
If, however, you decide to risk it and not get a UK license ... I'm guessing nothing will happen. Anyone?