You're probably having difficulty finding a service for this because there's no money in running one since Companies House announced and then launched the public beta of this wonderful free API
I couldn't see any way of querying the service by any kind of ranking so if you dont already have a way of getting the list of companies you want to check for then you might need an additional step first.
If you have the name of the company you want the document for, you can use the main api search to get a list you can select the company from, that return includes the company registration number
The Company number will let you use the filings api , to get a listing of all the filed documents, which includes a UID number for each document that can be used with the documents api to get the actual document as a pdf. They mentioned at the api launch that they see all of these documents being filed online in the future and then the information will be available through the api as JSON rather than or as well as the pdf, some filed documents might either be data or a file so the documents api can also return document metadata which returns a mime type for the document so you can check that your call will return what you expect.
If you dont have the names, one way o finding them might be to use the two data download services which let you get big chunks of data
The Companies service lives here and gives you all the data for live companies on the register, updated once a month:
And the Accounts one is here, this lets you grab all of the activity day by day, as well as historic data from last year:
Additionally, the accounts data in the download is in some kind of xml so that might be more useful to you.