If you want to take this opportunity to visit North and South America during one trip please look for travel dates till 26th of June 2015. Max stay 30 days. Valid for bookings made till 25th of February 2015 or until sold out. Do not forget to issue US tourist visa or ESTA certificate prior your flight to/via America.
Booking samples
UK - Santa Cruz from Ł568
UK - La Paz from Ł569
Spain - Santa Cruz - UK from Ł437
Amsterdam - Santa Cruz - UK from Ł436
Spain/Amsterdam - La Paz - UK from Ł463
Spain/Amsterdam - Bolivia - Germany from €727
Spain - Miami - Bolivia - UK from Ł450
Amsterdam - Miami - Bolivia - UK from Ł454
All these European cities are well connected by many low cost airlines. To make a basic idea how to reach them from your hometown you may use service of Skyscanner.
Booking sample of an open-jaw flights to Bolivia Ł436 here. In this case you would be flying from Amsterdam to Santa Cruz in Bolivia on 7th of March. Your return flight from South America to London would take place on 23rd of March 2015.