O2 UK prepaid SIMs used to have 0844, 0845 and 0870 numbers included in the 100 free minutes package that you got on the first top-up in a month, but this was altered a few days ago, hence the 15p a minute charge mentioned above.
The international bolt-on was free for a promotional period, but now ?4.99 as you mention.
I've previously suggested that people mix callthrough numbers from the 100 inclusive minutes and calls at those int'l cheap tariffs from the top-up credit, thus getting about 250 or 300 minutes for ?10. Now that 0844 etc are not from inclusive minutes, I'd suggest calling card or callthrough via London numbers, eg from Nobelcom or Call18185, 1899, or 18866. Note that the latter have different rates from BT landlines to those from mobiles and other landlines
http://www.call18185.com/mobilerates.php
http://www.call18866.co.uk/mobilerates.php
http://www.call1899.co.uk/mobilerates.php
http://www.nobelcom.co.uk/
There are also some operations where you sens a premium rate text message to top-up an account, then get calls against this credit, like Topupdiscount as mentioned
For more ideas about various methods, see call comparisons at
http://callchecker.moneysavingexpert...ntcallchecker/
http://niftylist.co.uk/calls/to/poland/landline/