As I stated earlier,
Being UK citizen does not always mean you are UK Domiciled for UK tax purposes. UK Domicile is inherited on birth from English/British father, generally, called 'Domicile of Origin' unless you have opted for UK 'Domicile of Choice' which is less likely.
Based on the above, you may wish to confirm yourself if you are UK or Pakistani Domicile.
Internet Business in the UK:
As you said you have despatch office in the UK, HMRC is likely to argue that you are 'trading within the UK' (as opposed to 'trading with the UK'), and so UK business profit is likely to be within the scope UK Tax - regardless of your Domicile status,
unless you can demonstrate that you have no permanent establishment (PE) of business in the UK - for which, for example, you collect all orders over the internet, run internet business (process the order, receive the money etc) from outside the UK and despatch the delivery directly from outside the UK.