Easy, "top" mosfet (connected to the B+) drops about all the voltage from B+ to the Ua (anode voltage). It`s about 100V difference, and with 15 mA - P=1,5W, even small heatsink is "good enough" - IMO. Interesting, in BC55B/MJE350 CCS, the transistor close to the anode load, or "lower" one (MJE350) needs cooling - opposite then DN2540 cascode CCS.