@phildenholm: You're right about them being banned, but only in certain circumstances.
Since June 13 this year, it's no longer allowed to only have a helpline number which would cost more to call than a call to "a friend or relative". That means that 01, 02 and 03 are allowed, as are 0800 and 0808 plus mobile numbers (07), but most other numbers are not.
However - this only applies to "helplines" for customers who want to discuss contracts which have been "concluded", ie. something you've already bought. An example here would be if I bought day tickets and had a query about them, or if I booked a hotel stay and wanted to change or cancel it.
At the moment, Towers are probably breaking the law here, as their contact page does not "prominently" list any free numbers for queries to existing bookings.
Of course - it would be within the rules for Alton Towers to have a free number for queries with existing bookings, and a chargeable number for new bookings. However, the logistics involved in maintaining such a split, along with the cost of handling calls to the free number that should have gone to the paid number, means that it's likely easier just to have a single number.
What's more, it's debatable what the Friends & Family Booking line would be classified as: whilst it could be considered a way of booking new tickets (therefore it could be charged for), it's quite clearly advertised as a perk of the Annual Pass that you've already bought (therefore it should be free).
Here's the detailed regulations (PDF) - the relevant sections are D(17) and the entirety of J.
Hope this clears this up (and Towers get their act together!)
*puts away legislation for the evening*