The operator Voipfone can call 0844 numbers. I was a little surprised to see the tariff as 8p + vat when I expected 5p + vat; maybe I remembered wrong, or they had to increase it.
You can set up a prepaid account with £5, and use it in a choice of several ways:
1. Using VoIP via a softphone in the computer, with headset and microphone or plugged in phone handset. Or a wi-fi phone with the account settings in it.
2. They give you a free 0560 and optional free 0870 incoming number; then you could forward your account to the 0844 number and dial your number from your home. But maybe reaching the +44560 would also present difficulties from abroad. And it still costs you that int'l call.
3. You could call Voipfone's London callthrough access number, from a recognised caller ID registered in your account, then dial the destination.
4. They have a callback system, which you can use online by logging in and entering the 2 numbers you want to connect together, i.e. your home number and the destination. There are also 2 callback method options for use from a mobile
5. Again after setting up call forwarding in your Voipfone account, you can call your account via Sipbroker access numbers in Spain that act rather like a callthrough number, though the onward number is a different format. Dial the access, then either *35030xxxxxx where 30xxxxxx is your 8-figure Voipfone account number, or *01344560xxxxxxx where
(0)560xxxxxxx is the incoming phone number for your account
I realise this all sounds complicated, but I'd suggest the last one, as it's access via a Spanish landline, then just dial your number forwarded to the destination
There are rather more complicated methods still, but unless you want to start calling many destinations via the Sipbroker numbers, this is enough for now.
http://www.voipfone.co.ukhttp://www.sipbroker.com/sipbroker/action/pstnNumbers - scroll down to Spain
http://www.sipbroker.com/sipbroker/action/providerWhitePages - this lists all the provider prefixes, which you don't really need, as I've already given the *350 and *013, but in case you or someone else finds other VoIP providers that can reach 0844 numbers a bit cheaper
It's a solution for now, if you aren't given a geographic number for the surgery ..... if it needs more explanation please ask