The more I learn about the economy the more I don't understand so I asked someone who understands these things to explain it in the most basic of basic terms and here is their reply.
In very basic terms being the holder of the reserve currency allows you to buy things cheaper than anyone else as they have they added cost of changing their money into your currency first.
For example, if a 42 gallon barrel of oil costs $6, America gets it for $6. For that same barrel, the UK has to pay $6 and $1 to change the pound into dollars first.
So America can sell that barrel of oil to the Americans for $14.28 per gallon. The UK has to sell it at the equivalent of $16.66.
So cheaper petrol costs means it is cheaper to deliver something 10 miles in America ($142) than in the UK ($166) so the item being delivered, once delivery costs are factored in, is always going to be cheaper in the US. (Taxes, inflation etc must also be considered but ignore those for this purpose)
Now imagine the pound became the reserve currency and it got its 42 gallons for £6 while America had to pay £6 plus £1 to convert its dollar into pounds first. Petrol prices should fall in the UK while prices would definitely rise in America and the cost of delivering the items by ship, plane, car van etc would rise due to the added cost of the fuel.
So basically, the reserve currency allows you to be cheaper than anyone else in the World so prices are kept down. Take that away and prices go up, budgets get squeezed, jobs are lost, unemployment rises and Government raises less revenue and basically it all unravels.
It's easy when someone explains it to you without all the blinding economic talk.