Look at points 90 and 91
Highway code
It has actually been clarified dont know the case law though, but if a person pulls out in front of you and there is an accident and they claim that they pulled out because you flashed them it is not a defence and they will be put at fault.
Driving is downto your own judgement and you should make decisions yourself.
Its the same as indicators you should mke sure that some one is actually turning before you pull out just because they have indicator on doesn't mean they are turning, if you pull out when they are indicating and there is an accident it will be your fault not the person indicating.
And yes i use Auto very good idea, who cares what other people think if lights go on for tunnels and bridges, you shouldnt give a shiit what others think, only opinion that matters is other lexus drivers lol and we know why you did it :-)