Mode a is completely useless to TCAS it does not report altitude as I understand
Not so. TCAS will still plot the target in azimuth and issue a T/A. You certainly should not take avoiding action based soley on this, but it is a tool to try and aquire the aircraft visually.
Lots of Mode A only aircraft in the UK, and you quite often get T/As from aircraft that are seperated vertically from you.