This circuit will operate in the following manner. As the current increases at some point the Q2 transistor will start to turn on. However the exact point at which it turns on will vary with temperature and will be nonlinear because it will turn of Q1 so quickly it that Q2 won't have a chance to get to the linear region. In the case of a large overload this will not be a problem because Q2 will be fully bias very quickly.
By add some resistance (try 10K) between the collector of Q2 and the gate of Q1 it will require a higher current to flow through Q2 to turn Q1 off. This will produce a cleaner current reduction point. However this current limiting method will start limiting at some point but won't shut off completely until a higher current is present. Off course when Q1 shuts off the current will fall and Q1 will turn back on thus creating an oscillation. This oscillation can be dampened by putting a capacitor between the source and gate of Q1. This may very well suit your needs as it has a more predictable current limit. If you are really implementing a 0 to 300 V and not just a 300 V power supply you will need a voltage regulator to go with this as this circuit is really just variable resistance power source.
Also keep in mind the voltages that components will have to withstand. VR1 has to withstand 300+ volts. Most reasonable sized pots will not handle this. This can be remedied but putting a 1/2W 50-100K resistor between the pot and ground. It is likely that the lower part of the range for the pot is useless anyway. Additionally R1 will be subject to nearly the full voltage if the load is great enough to trigger the current limiting. Using a 1 watt or 2 1/2W resistors will help to ensure this resistor can handle the voltage.
A fuse on the hot side of the primary is extremely important. I would also recommend a 150K 1 W resistor across C1 will with discharge the capacitor when it is unpowered which will make the circuit safer. Using a 10 OHM 3W resistor between one output winding of the transform and the bridge diodes will limit the surge current into C1 to safe level for the diodes and allow the fuse to be rated closer to your maximum current rating without blowing the fuses when applying power with C1 fully discharged. A MOV across C1 can also save Q1 from damaged due power spikes from the power line. The 10 OHM resistor will help keep the spikes contained and improve the life of the MOV. The reason I am making these suggestions is that high voltages need to be handled with extreme care. Resistors are cheap but they must be operated without their capabilities. Read datasheets for everything you use to be sure you understand all the limits of the parts.
If you are trying to maintain full output voltage and turn off quickly when the current limit is exceeded then you need a circuit with a non-linear but well defined switching point. The easiest way to do this is to use a comparator circuit. The tricky part is how do you want it to behave once the current limit has tripped. As with the circuit above most current limiters will turn back on once the current limiting kicks in. So if you want the circuit to stay off until power is removed you need a circuit that latches it's state. Another possibility is to have the power cut off and then turn back on after a delay.
High voltages complicate things and thus provide a challenge to keeping things simple and cheap, but this chip (STFC01) is design for use to around 50V but can be easily used for much high voltages using the techniques shown in the data sheet. This solution is very flexible but as I could not find a source for the chip so great idea but not happening. I did a little looking around for another device but did not find one that worked with high voltages.
If I knew what you really need this circuit to do for you I could give you some better information. If you needs are simply enough the circuit above with the tweaks I suggested may do what you need. If you needs are more complicated then you may want to consider entirely different solutions such as a DC to DC convert type circuit similar to what is used in computer but with a high voltage output instead of a low voltage output. The advantage of this type of power supply is it is efficient and there are several chips that do most of the work on the line voltage side and provide both regulation and current limiting secondary side. It does require finding an appropriate transform for the job but since the transforming with be small the cost should be reasonable.