Having it defined as ‘reasonable’ is so ambiguous. Different people are different people, and the body has different levels of physiological arousal.
What I, as a teenager with Asperger Syndrome, an anxiety stress disorder, consider a ‘reasonable’ response to the fear of a masked burglar in my apartment, is going to be different to a depressed person who’s given up caring would consider ‘reasonable’ in the same situation.
Also, different political affiliated people will have different opinions on ‘reason’, so is the UK’s self defence law too ambiguous and in need of clarification?