I hold both Australian and UK citizenships and passports as does my daughter.

I recently lodged applications to renew both passports which has to be done through VFS.

I have a brother who lives in Australia who also holds dual citizenship and passports.

Last week he lodged an application to renew his UK passport in Sydney which he was able to lodge online with the UK passport office at a lower cost and no need to attend the VFS office for lodgment and collection.

Any idea why two different requirements ?

In many countries you can apply online, then send your passport to the UK for renewal. The story in Thailand is that as it is a legal requirement to have your passport accessible at all times they accept a copy instead of the original passport. Presumably the copy has to be verified as such by VFS otherwise there would be no reason why you couldn't send it yourself. VFS also perform the cancellation of the old passport on collection of the new one.

Thanks for that response in my case I use my Australian passport in Thailand for the retirement extension but obviously I am in the minority.