I wouldn't say that they are scams, but I would be more inclined to go to a proper local firm, where you're actually going to have a solicitor acting on your case, as opposed to an unqualified case worker.
The 'No Win, No Fee' is based on insurance policies - either you will hold a Before the Event insurance policy via your motor or home insurance (check your policy and it should be listed as Home Legal Cover etc) in which you should contact the Legal Expense Provider in the first instance, as they usually decline cover to your own solicitor until court proceedings have been issued.
If your home / motor insurance does not include such cover, your solicitor will apply for an After the Event policy, where they set out the facts of the case, and must confirm whether they believe that the case has more than 51% chance of succeeding.
Use of these policies is only necessary if you lose your case. Your solicitor will then apply to have his fees paid by the insurance company. Also you may be required to pay the defendants costs, which will also be covered by the insurance.
Hope this clears it up some.