Why would there be passport checks while you were in transit airside? You technically never entered Germany. Would you have tried to leave the terminal to landside, you would have been checked by the border police. As you were not trying to do that, it was up to UK border control to check you leaving the country of departure (which the UK does not do, contrary to many other countries) and the Ukraine to check you entering Ukraine.
The UK, Ireland, and just a handful of other countries, do not have border control checking the passports of departing pax as there is no point. It is a waste of everyone's time but most countries do it.
The main concern is to check pax
entering a country and border control should have adequate staff to do the job. Queues are always long at border control in most countries because there aren't enough staff available most of the time, so why waste some of them on departing pax and make the queues in arrivals even longer?
Any country concerned about who is leaving can have carriers supply pax lists to their equivelants of the Home Office, or if they're after a specific person at a specific time, have the police at the gate or at security.