I renewed my passport 4-5 years ago in Bangkok at the British Embassy, however that passport is now nowhere to be seen. I've heard about a process of being able to renew or to replace a stolen / lost passport online as of this year? If so, what documents do I need and how do I exactly go about this? 

You can find info for getting a replacement passport here by going through the steps here. https://www.gov.uk/overseas-passports

You will need to get a police report for the lost stolen passport.

3 minutes ago, ubonjoe said:

You can find info for getting a replacement passport here by going through the steps here. https://www.gov.uk/overseas-passports

You will need to get a police report for the lost stolen passport.

Thanks a lot Joe