I was able to setup Google Voice and can now use that to send/receive texts for free.
You'll need an invite for Google Voice, and the initial setup needs to be done from a non-UK IP address (so you'll need a friend's help or to use a VPN). After it's setup, though, you can use the mobile Google Voice website and send/receive text messages that way.
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Skype is another option, but I think they charge per text, I'm not sure how much though. (I only use Skype for calling which is a flat rate per year.)
Hope that helps, PM me if you'd like more details on how I set it up.