If you can get some good statistics around your conversion, utilisation, penetration etc. for a data cycle, you will probably discover that you can get more out of your files by dialling smarter rather than harder.
The guidelines are available at the OfCom site (Statement on the persistent misuse of an electronic communications network or service aka The Commandments). I won't paraphrase it. I advise that you study this until you are fully conversant with the sections relative to the type of dialling you intend to do. Make sure people know that you don't (f)muck around when it comes to OfCom.
There are restrictions around how many times you can call a number in a day, depending on what technology you are employing and what dispositions have been applied to that customer record etc. You need to study the document.