Best guess UK pricing based on Canadian pricing an Roadster precedent
Based on volker.berlin's contributions and the Canadian 'precedent' and the UK Roadster pricing I arrive at approx. £75k for the Sig.
(Typical Roadster price £101,900).
Am I far off base?
A quick-'n'-dirty ratio comparison of Sigs to Roadsters in other markets says you're very close. The RHD penalty should be about the same.