We are unfortunate for having so many systems of measurement. This is not the case only with energy. Consider length, or mass. We have metres, miles, feet, inches, kg, ounces, pounds, and so on.
But just take the SI system as the official one, and you will have one basic unit for each quantity.
Why don't you use miles to weigh yourself? And why don't you order a metre of milk? We have different units to quantify different types of measurement, although (as Dex pointed out) they are often related, and you can usually describe one unit (Volt, like on a battery, for example) in terms of other units.
Good luck with your GCSE, stick at it and if you can do an AS in Physics then you'll have a better chance of getting into uni!