The expenses I need to cover are housing, transportation, food, and screen-printed clothing and backpack. 

My intention is to go as cheap as possible. 

The accommodation would be in a hostel about 30 € per night. (3 nights = 90 €) 

Moving to London would be in a flight from Asturias Airport. Approximately, go and return could go for about 200 €. 

The majority of these hostels are bed&breakfast, which means that breakfast is included but not lunch and dinner. About 10 pounds per meal, £20 per day. (25 € for 4 days = € 100) 

To move from the airport to central London, you need to take a bus which costs about 8 pounds each ticket. (20€ go and return). 

And to move through the different parts of London, the fastest way is the subway. Reading forums, I could go for £30 the 4 days, including the Oyster card. 

The screen-printed clothing I don´t know exactly. In other times, screen-printed sweatshirt has cost me 20 €. So an estimation would be about 80 €. 

The total of all these needs is about 520 €. This is estimated. 

I have about a month to get the total amount, because if I wait longer, the price of tickets or hostel could increase or have no space directly.