Few points to be noted...
1- When a vehicle arrives at any port in pakistan and goes through the process os clearance, it gets a vehicle clearance certificate from customs and excise. Now I tell you what this clearance means though it is not a paper certificate. pakistan customs are linked with the Interpol's database of stolen cars. All they do is feed VIN in the system and then know immediately that whether vehicle is stolen anywhere. If a car is stolen in UK, its VIN and engine no goes to Scotland yard from where it goes to Interpol's database.
2-This is true that even stolen cars get cleared through various ports of pakistan but there is no risk for the purchaser because there is no agreement between UK and pakistan for stolen cars and most importantly once vehicle is cleared after paying taxes, noone can take it. You will be well aware of the term superdari through courts.
Basically if a stolen vehicle is cleared after paying taxes, liability goes to customs and excise. What can they answer in court that why they let the vehicle cleared, and if they make any excuse, they would have to pay back all duty. So think what kind of hasle it would be for them.
Most mercs and bmws come from UK, reason is very simple and that is right hand drive, rest of the europe is left hand drive.
Before buying any car make sure you sign the agreement on Rs 50 stamp paper and keep the proof of purchase. It will save you rmoney if wost comes.