Hi
If the Hong Kong Severe Disablement Allowance is similar in nature to the social security benefits at EIM76100, then it is not taxable and should not be included in a self assessment tax return.
Take a look at the guidance here
EIM76100 - Social security benefits: list of non-taxable social security benefits
However, any foreign social security benefit that is also a pension, must be considered a foreign pension and included in a self assessment tax return (
EIM76009 - Social security benefits: taxable foreign benefits).
If you need to delare this as a pension, it would be on SA106 page F2, in the section for pensions (
Self Assessment: Foreign (SA106)).
Thank you