

You don’t necessarily need full sun any more (though it helps). Solar panels are a lot better than they used to be at generating power on cloudy, overcast days. You can even use them in the North of Britain these days. Of course they generate more on both of our sunny days, but they still work on our normal rainy cloudy days too.
I remember reading about some plan to fill the Sahara desert with solar panels, which would in theory be able to power the whole world, but I imagine there’s a whole lot of politics, profit and greed in the way of something like that ever working (I imagine there’s also a tonne of engineering and environmental considerations).

In the UK, the lowest guess (from the government) is “more than 10,000”. Most anti-slavery charities agree on an estimate of about 120,000 - 130,000 in the UK.
Each year, about 10,000 - 20,000 are newly referred to authorities (and potentially rescued).
Though there’s often a perception that it’s only foreign workers in the UK that get trapped like this, 25% of those are British nationals.
Some sad (but important) links: antislavery.org
unseenuk.org
The Modern Slavery Act 2015: becoming world-leading again (PDF)