1) Look at the For Reference topic on how lighting circuits work
2) Buy a book, e.g. the Which? guide to wiring and lighting
3) Get an electrician in.
But whatever you do, also -
a) turn off the power and make the live wires safe with choc-block.
b) get onto your solicitor - luminaires and switches are fixtures and fittings, and it was illegal for your vendors to remove them. Get him to write a heavy letter making it crystal clear that you will not hesitate for one minute to take them to court if they do not immediately, and at their expense, get an electrician to come and replace what they removed.
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