118 118 calls cost about 98p assuming they last for less than one minute (69p fixed + 29p per minute), they are now the biggest directory enquiry service by use of heavy expensive television advertising.
They also have the number 118 811 which is considerably cheaper at 40p fixed
I suggest you use one of the following (if indeed you need to use any)
0800 100 100 - free and financed by 20 second advert
118 226 costing about 23.5p fixed
If you use none BT telephone services, then 118118 is sometimes even dearer!
These prices are frequently changing so I believe all the above is reasonably accurate.