Well it is the 1st of June and this information is now in the public domain so here are the 'Italian' prices to give you a good indication for the UK.
The current A500 €18.800,00
A500C €21.000,00
So €2.200,00 more for an A500C (this pretty much reflects the £2,000 premium of 500C's over equivalent 500's)
Also i know this is the 500 section but the Abarth Punto Evo lists at €19.400,00 i belive the GPA listed at €18.100,00 so €1.300,00 extra for MultiAir + other goodies.