I already love 18185 for my landline calls. It's a flat rate 6p per call any time of day. (I use BT's option 2 for evenings and weekends, but their daytime rates are extortionate). I use it for international calls that are either 1p or 1/2p a minute, but I've just discovered that they now offer 1p text messages to mobile phones.
For me, the appeal is not just the low price, it's the fact that I can send the message from my computer - I find it much easier to touch type in front of my computer, then to peck at the keys of my mobile.
But as I have a pay as you go mobile and the cheapest text is 5p, the cost is an added bonus.
If you want to
give it a try, there's no rental cost, it's just a cost per call billed to your credit card once a month.