If you want to take this opportunity to visit North and South America during one trip please look for travel dates till 30th of November 2015. (except blackout period during summer holidays). Max stay 30 days. Valid for bookings made till 17th of April 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 - Bolivia from £561
Spain - Bolivia for €598
Spain - Santa Cruz - UK from £425
Multi-city booking sample (USA + Bolivia)
Spain - Miami - Bolivia - UK from £424
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 £424 here. In this case you would be flying from Barcelona to Miami on 8th of May. You would the fly from USA to Santa Cruz in Bolivia on 14th of May. Your return flight from South America to London would take place on 23rd of May 2015.