We recommend for you to get in touch with CHD. Cultural Heritage Digitisation Ltd (CHD) are a specialist scanning company based in Wandsworth, South West London.
Amongst many scanning services they have a large format Cruse Scanner and can digitise large flat artworks of any media type. They will be able to provide you with a high quality flat RAW file.
Please find their contact details below.
Website: http://culturalheritagedigitisation.co.uk/
Email: info@chdigitisation.co.uk