As a basic rule anything you pay AUS $ for here in Australia will cost you the same in British pounds in England. So $10 worth of goods here will cost you 10pounds there.
Cigarettes are far more expensive than here.
London is one of the most expensive places on earth so be careful where you stay and what you do.
As a 21 year old, look at joining YHA(Youth Hostels Australia), which has a British equivalent. While you do not have to stay at youth hostels, they are the places where people with less money travel, and they learn all the best places to find value for money.
They also have made deals with places for discounts that you can access through membership.
An additional idea is to try and contact people in the UK in the area you are going and ask them for the cost of various things.
Seeing how you have already planned your holiday I am sure you will do very well.
No matter where you go there are basic rules that will increase your spending.
Smoking costs more.
Drinking alcohol costs more.
Paying for other people increases costs.
Buying tourist gifts and presents to bring home increase costs.
As long as you have moderation, $5,000 spending money for a month would be great. You will have rest days where you spend less and you will have big days that are very expensive.
Check the internet because many of the events you want to see and do will have prices online. Eating food while you are out will cost more too.
I went to Stonehenge. It was cheap and a great experience.
Good luck.