Would you have any other suggestions?
Sorry this is an old thread, but I only just joined. Yes you are right that Talkout is good for the zero connection charge and their rates are half decent. There are a few others of which Lebara.es is quite well known and aimed at the immigrant market. It allows you to call UK at a cheaper 8c/min but there is a 25c connection charge!
Masmovil is similar but with an even higher connction charge.
Incidentally, many Spanish MVNOs (inc.Simyo) will also let you make calls to Spanish mobiles at around 8c/min, so you could call back to the UK via the callthrough you suggested at this price, which is the same price as Lebara and many others, but with smaller connection charges. In my opinion, Lebara Spain is a bit of a rip off especially for calls to Spain at 15c/min with 25c connection.
Personally, as I make a lot of calls back to the UK from my mobile, I use a Spanish Simyo SIM, pay 5 Euros a month for 500MB of data, which if I want allows me to make around 500mins of calls back to the UK via a Betamax account that offers freecalls to the UK. (You will need a SIP capable handset, or 3G modem and laptop). More info in that other thread!