To be honest as your already well along the BSAC training system why not look at the BSAC ERD Course after you have completed the Combined Nitrox Course.
The BSAC ERD gives you exactly the same qualification as TDI ADV Nitrox/Deco Procedures. In fact if you want to be picky it actually gives you a little more as the BSAC course gives you a 55m qualification whereas the TDI only gives you 45m.
Check out the course here:
Extended Range Diving - BSAC
After this you could then look at the BSAC Sports Mixed Gas Diver Course etc
Just to let you know though I did the TDI combined courses and they are excellent courses as long as you have the right instructor as the manuals are crap.