I don't know the service, so I was surprised to receive a spam text message from them a couple of weeks ago.
I think the O2 bolt-on is already cheaper to Poland landlines than dialling via a London callthrough number.
But I might suggest changing network. There are some other callthrough arrangements that have been subject to a number of changes of network policy, and now I'd say using T-mobile payg has recently gained a very attractive option for calling abroad.
http://www.yourcallworld.com/tmobile/
There are a range of new 07755 access numbers for different destinations - 3 pence a minute to many landlines (and US mobiles), 10p a minute to Polish mobiles, 15p most other European mobiles. No account needs to be set up, and these charges are independent and complete, not on top of a T-mobile rate (I've checked: 1p minimum charge).
T-mobile also has pretty good rates for UK mobiles, at 12p/min. This rate applies to UK landlines too, as these are excepted from the YCW service. The Finarea callback service ReturnCall might be useful at 2p/min UK landlines, and 5p/min for UK 0870 numbers