I don't know of any place selling peonies in the UK. In the US for peonies you could try the
following:
http://gilberthwild.com/
http://www.naturespromise.com/
http://www.goldenport.com/peony/
http://www.greatplants.com/

 www.arenaflowers.com/flower_delivery/florists_in_england_uk  
 
www.garden-marketplace.co.uk/garden_plants/peonies.html  
 
as you share the same name as me figured would help  
 
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