I don't think it's a good idea to quote the cost comparisons in pence per minute, for two reasons:
1. Some people will see it penny-pinching. They will view it as a trivial issue about "a difference of only 4½ pence", rather than the huge amounts being creamed off by the call recipients.
2. A lot of people over-estimate the true cost of phone calls. When they see a price of 7.51p/min, some people will think that's quite cheap. These kinds of people typically make heavy use of PAYG mobiles etc, and are used to paying much more for their phone calls. They'll think it's a big fuss about nothing.
I think it's much better to say something like "Calls to 0870 numbers are surcharged by at least 150% on top of normal national call rates and the organisation you're calling receives most of that hefty surcharge". Talk in percentage terms, not in pence. The impact is much greater. As soon as you start quoting pence per minute, it will trivialise the issue for many people.
I have a paragraph about the scam on my web site at
http://www.nicholashodder.com/scams.htm#0870 . Please feel free to copy any part of it for a viral e-mail.