The UK check UK bound (international) flights as well. As do the US. The reason why some don't bother to check departing passenger is explained above. Both the US and the UK use Advanced Passenger Information System and have the identities of departing passengers transmitted by airlines to check who left, who stayed, etc.

As far as EU member states go, the Schengen countries don't do any immigration checks for flights within the Schengen area. If it weren't for airlines (that want to see if the passenger is actually the one in the reservation), you could just walk up to the gate and board a Schengen flight. Only flights that leave the Schengen area will be subjected to an exit control (so no different from other nations).

I didn't not follow the debate regarding the Passenger Name Record (PNR) that will EU member states will have to transpose by May '18, but I doubt that it will change much to the current form of exit control, which is - globally speaking - still very common.