Although Crimestoppers receives funding from the Home Office and individual police forces, it's actually an independent charity. While, for example, calls to the emergency services (i.e. on 999 or 112) will always show the number you're calling from, Crimestoppers can't see the originating number for an incoming call if you dial 141 first. (Actually, you don't need to dial 141 because Crimestoppers guarantee that they'll never try to trace a call, but dialling 141 first gives some people added confidence about retaining their anonymity). If you still didn't have complete faith in their system, you could always make the call from a public call box.
Crimestoppers pay a reward, of up to �1000, if the information they receive leads to someone being arrested and charged. (That effectively provides feedback, as you'll be invited to call Crimestoppers to see if someone is facing charges as a result of your call). Your anonymity is retained through a code system. See here:
http://www.crimestoppers-uk.org/giving-informa tion/rewards
To the best of my knowledge, there's no direct feedback available if you report benefit fraud. Investigators are obliged to retain your confidentiality but you can make an anonymous report if you prefer. Once again, if you're really worried that someone might try to trace you, simply use a public call box. Alternatively, use the online form. (You couldn't be traced unless investigators obtained a court order to make your ISP reveal your details. Since you're not suspected of any crime, they couldn't do this. If you're really paranoid, use the online form from an internet caf�):
http://www.direct.gov.uk/en/MoneyTaxAndBenefit s/BenefitsTaxCreditsA