Your best bet would be horse manure. This can be used fresh if need be and is readily available and the right price.
Picture shows garlic and broad beans top dressed with manure that was fresh from the stables the last week of November, stored for 6 weeks due to bad weather.
As for using peat: In London this is not a resource, this is asset stripping. For bulk materials although deals can always be done, you should get used to thinking £40 per ton for any material of nominal value.