am I eligible for direct transit

Hello, am I able to enjoy direct transit exemption for travel from Uk to Shanghai with airline A, collect luggage after passport control, then re-enter immediately to fly to Tokyo with airline B three hours later?

Also, if this is possible, can I get my boarding pass for the Shanghai to Tokyo flight before passport control as I need to show a "seat allocation" to enjoy the direct transit exemption.

Many thanks for your help