Since it says under your username that you're an English expat you might be interested in a little British company called Localphone.
I think they started out as a virtual calling-card company but once signed up you can add a VOIP account. I know you said you only wanted it for calling out but Localphone give you a free local UK number (none of those lo-call rate thingies) from any area-code you want with your VOIP account, which beats the $35/year Gizmo charges. The call out rates are pretty reasonable, with pretty much all of them being cheaper than Gizmo the last time i looked.
A couple of features i like about the service that may not be useful for you unless you come back to the UK often and then want to make calls abroad are the 50 local numbers you get that you can assign to any other phone number in the world.
For instance, i have a local number mapped to my mother's home phone number in Portugal. This means if i'm not at home within range of my SIP device i can simply call a local number using the inclusive minutes on my mobile and it will ring my mother's phone in Portugal and i'll only be charged 0.8p per minute by Localphone. And you get 50 of these numbers, plus a bog-standard calling card number in case you need to call others. This also has the benefit of getting around the mobile companies blocking of calling card companies, since there really is no difference between a real local number and the virtual redirect numbers.
Both the calling card and VOIP rates are the same, and you only pay the call charges, there's no set-up or monthly fees (you can even just signup and use the free call in number if you want).
Anyway, i'll stop before i start sounding too much like an advert, and just provide a couple of links. ^_^
Localphone (with referral, in case you want to reward me)
Localphone (without, in case you don't).
Edit: Sorry, i didn't realise the original post was from more than 6 months ago. The OP probably has a solution by now.