While big businesses can send texts from landlines, Joe Public can't.
That's because you need a special text-enabled phone, which has to be registered with BT Text (or a similar service operated by another company) before it can be used.
You can send texts from a computer though. There are various services available. This one, for example, gives you your first 10 texts for free and you can buy additional credits after that (starting at 5p per text if you only buy a pound's worth but getting cheaper if you buy in bulk):
http://www.sms4free.co.uk/