You can get traveler's cheques. Banco Piano issues them, and it looks like they have a branch in Rosario.
http://www.bancopiano.com.ar/sucursales.aspx That is surely the cheapest way. Also, Banco Piano will transfer the pesos to any account anywhere in the world via Money Gram for 2% commission. You do not have to have an account with Banco Piano for that either. Just show up with your passport, cash in pesos and bank info in the UK (name, address, swift code, acct #, etc).