I faced the same problem when I tried to add energy monitoring in my home which is also an UK-style electrical box. Here is the electrical box in my recent IotaWatt installation.
I ended up using the solid core CT - “DL-CT08CL2" (1000 turns). The IotaWatt unit comes with burden resistor value of 24 ohms, which translates into around 48.6 Amps of maximum RMS current. If you know a little bit of soldering skills, you may change the burden resistor to increase the sensitivity.
Also, the DL-CT08CL2 does not have the input jack, and you may need to extend the wire and add a input jack by yourselves.
Reference:
https://www.alibaba.com/product-detail/Current-Sensor-DL-CT08CL2-DL-CT08CL5_1600153956920.html
https://tyler.anairo.com/projects/open-energy-monitor-calculator