If you can't keep below the 400000 pesos threshold, keep in mind that even as "responsable inscripto", when exporting a service you are exempt from charging IVA, so that might offset somehow some of the costs of paying ganancias. For example, instead of charging 100 + IVA (121), you can charge 115, basically splitting "ganancias" between you and the client.
In my case, it was a no-brainer to pay taxes here instead of in New Zealand, but given that UK has a tax agreement with Argentina your situation could be different.