The UK has a very large energy industry. Electricity is a primary resource.

in 2014 the UK power generators (power stations) produced 335 TeraWatts hours of electricity.

335 TeraWatts Hours= 335,000,000 MegaWatts hours

Now to put a monetary value on this is quite simple. The average price for a delivered megawatt hour is about £100 per MegaWatt hour.
Yes £100 MW/h

Thus:-

335,000,000 Megawatts x £100 = £33,500,000,000

Yes, that is £33,500 million = £33.5 Billion.

That is equivalent to 2.23% of UK GDP.