The top 5 practices Accenture. KPMG, Pricewaterhouse..,PA consulting, Cap gemini, in fact all the usual players will (IMHO)
fill this space. Others include Knowledge Associates and some other smaller niche companies. Change management is not as big in the UK as USA.
Much depends upon what you offer and if it has associated tools eg, Knowledge management tools. There are some good sources of information available to help you set up in the UK - If you email me a bit more I could help by pointing you in the right direction.