Calling 118 118 (UK) - A market failure? Is the shocking cost of directory enquiry numbers in the UK a market failure, or the economy working perfectly well?

EDIT: I know that proponents of free markets often like to blame over-regulation or government interference for perceived problems in markets, so I wonder if anyone could point to some regulation that has caused this incredible series of price hikes.
When there was only one telephone service supplier, BT directory enquiries used to be free. If you did not know the number of who you wanted to call, you could not call. Once you got the number from the phone book or directory enquiries, you could make the call and would be charged for it - instant payback for the phone company.

After the telephone market was opened to competition, BT started charging for directory enquiries eventually reaching 40p per call.

After de-regulation of directory enquiries in 2003, hundreds of suppliers have sprung up. The cheapest is currently 40p per minute plus your phone provider's Access Charge. The most expensive is £15.98 per call, plus £7.99 per minute after 60 seconds, plus your phone provider's Access Charge.

Prices for a number of 118 services have risen substantially over the years.

118 118 prices have risen sharply in recent years...

1 July 2015 - £5.74 per call, plus £2.99 per minute after 60 seconds, plus your phone provider's Access Charge.

1 March 2016 - £6.98 per call, plus £3.49 per minute after 60 seconds, plus your phone provider's Access Charge.

1 May 2017 - £8.98 per call, plus £4.49 per minute after 60 seconds, plus your phone provider's Access Charge.

It is no surprise that the most well-known services such as 118118 and 118500 are the most heavily advertised.

It should come as no surprise that the most heavily advertised services are among the most expensive. All that advertising has to be paid for somehow.

The cheapest services have no budget for advertising. In order to advertise, they would have to increase their prices.

What is needed is a third-party to rate and review all of the suppliers and their services and for this information to be freely available and easily accessible.