Is the salary received by a young Syrian refugee in Britain as a disability benefit considered haraam (forbidden), knowing that he was compelled to use crutches to demonstrate difficulty in walking in order to obtain it, while he genuinely suffers from severe pain and multiple ailments that prevent him from performing his work?

You must seek forgiveness for the deception to take what you do not deserve, and return to the competent authority to clarify the truth about your health condition, and to prove that illnesses prevent you from working, not the need for crutches. If they approve a disability pension for you, then there is no sin in it; otherwise, desist from that.