there is nothing wrong with twin entry valves, and usually they are easy to change to normal valves anyway, providing the rad is compatible. Why would getting a new system be the best option? after all , he wants to flush, thats all!! getting a new system installed when nothing is really wrong is financially stupid as a new system would not pay for itself for many years. unless of course the system is dangerous or not working at all and cannot be fixed. i just converted to c plan and already im saving cash, and i did not have to rip out the system to achieve this,
with twin entry valves on microbore, the flow side of the valve has a long pipe attached to it which runs along the bottom of the rad inside and sprays the water in at the opposite end to the valve. the water then circulates through the rad, back to the valve again and exits through the return side of the valve. might not be the best way of doing it but it does work as my system is the same downstairs and i have no issues.
the better question to ask may be, "what is wrong with your system which requires flushing?"