I'm no expert on pumps, but it looks like you have two pumps in series there (the main circulator and the UFH), and that's going to give you problems.
Also, you don't appear to have any bypass in the system. If you're expecting the towel radiators to act as a bypass, you should be aware that the 2 port valve on that half of the circuit will prevent them from working as such. Unless one of the other radiators is calling for heat, that valve will be shut.
There is to be a ABV/loop installed between the two parallel flow paths before the zone valves. its not shown on the drawing, but this will maintain the flow of water from the boiler while the zone valve are operating.
The towel rails will then act as the ABV for the heating loop while the ABV installed at the manifold will carry out this function for the UFH loop.