I don't know any travel agents in Argentina but..
1. You can try to book the Buenos Aires - Madrid/London flight with Air Madrid, they sell reasonable priced one-ways.
2. With all 'big' airlines you normally pay less for a return flight than for one way. In that case you can book a return flight B.A.-Madrid/London-B.A. (make sure to book it this way, because when you don't show on the first leg, they cancel the return!)
Hope this helps..