The UK / Hong Kong tax treaty allows for employment income arising in Hong Kong, when you were resident there, to be only taxable there.
So this income should not be declared (you can show it in box 19 of SA100, as a freehand note). Interest in Hong Kong that arises while you were not resident in the UK, is not taxable in the UK.
All overseas interest arising while resident in the UK, is taxable in the UK, so should be declared on a self assessment tax return.
A self assessment tax return should declare your 'world-wide' income, so strictly speaking, this is why you would declare non UK taxable income as a freehand note, so that it is not included in any calculations.
If the stock that was disposed of is from overseas, then yes it will need to be declared in a tax return. Article 17 of the UK/Hong Kong double taxation agreement advises:
"Pensions and other similar remuneration (including a lump sum payment) arising in a Contracting Party and paid to a resident of the other Contracting Party in consideration of past employment or self-employment and social security pensions
shall be taxable only in the first-mentioned Party".
This would include mandatory Provident funds (MPF) Occupational Retirement Scheme Ordinance (ORSO).
Please see UK/HONG KONG DOUBLE TAXATION AGREEMENT AND PROTOCOL