if you are a potential whistleblower or an investigative journalist or political blogger or even a police or intelligence agency handler of Covert Human Intelligence Sources (CHIS), you may well need to obtain or recommend, a hard (but not impossible) to trace, disposable "burner" pre-paid mobile phone, to arrange face to face meetings or document / data / money etc. dead drops.
If you have established initial contact with a potential confidential source or whistleblower, at least do them the basic security courtesy of not having to share a contact number with other confidential sources - an investigation into one such source, may very well draw attention to and betray the identities of other unrelated sources who have contacted the same phone number.
See our Hints & Tips for Whistleblowers etc. article: Buying a pre-paid phone card or mobile top up calling credit voucher anonymously
Tesco supermarkets are currently selling what they claim to be the "Cheapest Unlocked Mobile on the High Street" for under £10:
This is a very basic voice and SMS only phone (no camera, no removable SD memory etc.) but consequently with a long standby battery life (a claimed 560 hours i.e. over 3 weeks), so it could be used as a disposable Tracking device, if registered with a web map phone tracking service.
The choice of Free SIM cards (otherwise available for 99p each) is: Lycamobile and Lebara (often used for cheap international calls) and the main UK brand names i.e Vodafone, Orange, O2, T-Mobile and Virgin.
Tempting as it is to buy two or more such cheap phones (for the spare battery and power charger alone), do not do so from Tesco. They seem to have an arbitrary, unpublished or poorly advertised "rationing" policy in place, limiting the number of such "bargain" phones which an individual customer can buy at a time e.g. "one per customer in 24 hours".
You do not want the supermarket checkout operator to have to "consult" with the floor supervisor or have to argue about how many phones you are allowed to buy, not if you are trying to remain forgotten and anonymous in case of future whistleblower leak investigations.
Obviously if you are getting a trusted third party to buy the pre-paid mobile phone for you, this may be less of a risk to consider.