Every electrical item draws a certain amount of electrical current, measured in Amps. As is common to many campsite hook-ups, the system will trip out if the TOTAL current drawn by your electrical items exceeds 10A.
The current drawn will depend upon the power ratings of the appliances used. You'll need to check the labels of your appliances to find those ratings. The formula you need to know is that the current is found by dividing the power (in watts) by 230 (which is the nominal UK supply voltage).
So, for example, a 3kW (=3000W) kettle will draw 3000 ÷ 230 Amps = (roughly) 13A. So using such a kettle would cause the hook-up to 'trip out'. However some kettles use less power. A Tesco Value kettle (for £4.78), for example, is only rated at 2.2kW, so dividing 2200 by 230 shows that it would only draw 9.6A so you could use that kettle as long as you didn't have anything else plugged in at the same time.
Chris