You have two options here: either event monitoring or current monitoring.
For event monitoring, life is simple. You just put a lux sensor near the dumb lights and then when it comes on between sunset and sunrise you know those lights came on, and you trigger the smart lights off of that. No wiring required. There may be a slight lag in order to process everything, but for this kind of use case It’s usually acceptable. So first the dumb lights come on and then A few seconds later the smart lights come on. This option can work even if the dumb lights are battery powered.
Alternatively, you need to wire something into the circuit that can detect the change in draw. And that something needs to be able to communicate with SmartThings.
The Fortrezz Mimolite is a popular device for this purpose and is sold in both the US and UK frequencies. It works well with SmartThings. It’s just that it’s a bit expensive. People do use them to monitor all kinds of non-network devices, and it has a number of advanced features including being able to set a range of draw to monitor.
There should be some less expensive options but I’m not as familiar with the UK devices. The Fibaro door sensor does have dry contacts and might be usable (so that it would report open when the nonnetworked lights were on and closed when the nonnetworked lights are off, for example), but I honestly don’t know if it would work without requiring a transformer.
Hopefully someone who has more experience with UK devices can comment further on the options.