You could use Vyke to send an SMS.
Via their website it's 1.07p to UK or USA mobile.
Depending on your model of phone you can download their software to it and send SMS via your phone, same 1.07p SMS plus whatever small charge for data use on the mobile.
I also use their Call @ service whilst abroad so I can call home cheaply. I usually pop to a local phone box that accepts calls (I test call it first) then I enter that number to take the call on and enter the destination I want to call (usually landline to keep costs down), then using data it informs the system and shortly afterwards the phone box rings, I answer it then says please wait whilst we connect your call and rings the person I want to speak to, only cost a couple of pence in data if chargeable and around 2.2p - 2.5p / min for the call.
There's also
http://www.18185.co.uk/newsarchive.php#0 send from the website for 1p per SMS. If you make a quick call on your mobile using the access code and connect to destination (ie a chargeable call using 18185 via your mobile) you then get the option to use that number as the sender.