The US Dollar is woven into the fabric of Argentine life and is much more attractive for exchange than any other currency. To find out what's best for you will require a little research and a calculator.
Banco Piano in Buenos Aires will change GBP into ARS with very little fuss but often quite long queues. Check their website to find out if their rates of exchange are acceptable to you.
Clarin, La Nacion and other Argentine newspapers will tell you the latest USD exchange rates. Not so long ago I used to change GBP into USD whilst in the UK and change those dollars into pesos in Argentina. The benefit in presenting USD for exchange here far outweighed the double conversion costs. Check the difference between the two different approaches today and see if you think it would be worthwhile. In my UK locality, The Money Shop used to give the best dollar exchange rate and supplied crisp, new, sequentially-numbered USD100 bills which are premium for peso exchange.
Enjoy your stay!