Highway Code Rule 172:The approach to a junction may have a ‘Give Way’ sign or a triangle marked on the road. You
MUST give way to traffic on the main road when emerging from a junction with broken white lines across the road.
Laws RTA 1988 sect 36 & TSRGD regs 10(1),16(1) & 25
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The driver exiting the junction is 100% at fault. It doesn't matter that the other car has stopped or flashed its lights or waved him out etc, the car pulling out is obliged by the above laws to make sure it is clear to pull out befor manoeuvring and give way to all other traffic.
Yes, it would be good for the the cyclist to ride defensively and notice that a) there's a junction, b) that there's a gap forming and c) that another vehicle could be pulling out, but these only mitigate against someone else doing something stupid and possibly protect themselves from harm, none of them are obligatory.