Ofcom has said that the different pricing points that 0844/0871 can be set it is to allow for competition of telephone services on price. To have retail price competition with 0844/0871 numbers:

1. The price must be known (displayed) to the prospective customer.

2. The call rate must be the "deciding factor" for a customer to choose a particular service provider from its competition.

There is a good selection of dial-up ISPs and cheap dial through providers at different prices on these numbers and this is probably because the call rates are clearly shown. This is also the main income for these service providers from their customers (the callers). Customers of these services have chosen them on the retail call price of the telephone number.

For other types of businesses, such as mail order ones where 0844 is for customer services is not the main source of income, so the call revenue is relatively tiny. That's not to mention that this sort of use may be described as a "locked in" use. Add to this, customers may perceive that there is a low likelihood of calling such customer services numbers, if they have a problem with their order, for example.

NGMsGhost wrote on Oct 5^th, 2007 at 7:57pm:

But of course, there is a whole section on London numbers starting 3. See here. Note that this is in the under the heading "Information for Telecommunications Industry".

Ofcom has apparently given up on the "User's Guide to Telephone Numbering" (discussed here), which also, bizarrely was also under "Information for Telecommunications Industry".