Hi,
I also have AS and am now on anti-TNF and am have a UK Class 1(OML). From my experience when you start taking it you are unfit to fly for 4 weeks. From then on you will have a Class 1 with Operational Multi Crew Limitation(OML) - but the CAA medical section are of course the definitive. For what its worth I found the treatment did help but it is quite inconvenient.
360 auto feel free to PM more for more info.
theNUT