Butterfly bush cuttings
I have asked for some additional help from a horticulture specialist. But I can offer some resources which may help you in the meantime.
USDA Handbook 66 is a guide for post-harvest management of crops including nursery stock. They have a section on herbaceous perennials: http://www.ba.ars.usda.gov/hb66/herbaceousPerennials.pdf. This suggest plugs can be stored at 5 C for 3 months.
A guide from Clemson provides some general cultivation recommendations: http://www.clemson.edu/extension/hgic/plants/landscape/shrubs/hgic1084.html. This guide refers to USDA Zone 5. This is a classification of regional climate and includes minimum temperature as a main factor (http://planthardiness.ars.usda.gov/PHZMWeb/InteractiveMap.aspx). Zone 5 is -10 to -20 F (-23 to -29 C) so I suspect your climate is adequate given your regional data (http://www.metoffice.gov.uk/climate/uk/regional-climates/ns).