You need the dual channel hive to replace the UP2. From the UP2 instructions, you can determine the function of each terminal as below.
The wires should go to the corresponding terminals at the Hive dual channel version, which as you can see, match quite nicely
However for it to work, the existing wireless thermostat must either be decommissioned properly which would involve removing the grey and black wires from the thermostat receiver and connecting them together. If you can do this at their origin you can remove the WR1 thermostat received completely.
[The black wire from the WR1 goes to terminal 4 at the UP2, and from your photo, it looks as if the grey wire goes to the separate screw connector with a red wire.
If so, disconnect the 4 core cable going to WR1, remove it completely and put the red wire in terminal 4 of the Hive instead of the black one. If my assumption is wrong, post back what is connected where]
Or, alternatively, you could leave the existing thermostat in place and set it to maximum so that the 'on' light was permanently lit, which would mean always maintaining the batteries in the remote WR1 thermostat otherwise the heating would be open circuit and not operate when the Hive switches it on.