Wow those are nice blocks. Shame to break them up.
Regarding batteries, I'd recommend splitting in parallel with same amperage and voltage. So you wire both halves in parallel directly to the inverter or preferably through a junction box with separate pack fuses and even contactors. So you could run off 1 pack at a time in an emergency etc, or if one pack fuse blows you can run off the second etc.
I agree also if it is similar challenge that you split the pack and keep the inverter with the motor rather than move the inverter further away.
Regarding UK law, there are new rules I've not checked but the rule has always been any major changes to structure of the vehicle requires an IVA test same as building a vehicle from scratch. What constitutes major changes is subject to the investigator's opinion but you'll need a basic VOSA inspection to verify the conversion to electric so speak to the centre about any mod to the chassis ref mounting the motor in the tunnel and you should know what's achievable and how far to go. An MOT is always required first but that is easier with an EV.
Regarding mounting the motor in the tunnel and space, if you build a frame into the floor pan with strong box/tube section and load the motor from above your structure should be fine if my stronger. I'd tie it to the side rails of the chassis for added torsional stiffness. And mount the motor slightly offset to passenger side to have good driver leg room as a passenger only needs somewhere to rest their feet.
Any pics of the underside of the chassis would help also, perhaps with a tape measure in view for scale and reference and we can brainstorm a robust design for locating the motor. That is the biggest blocker to begin with because of six and shape in the small car.