Actually, I use this a lot. However, I'm in Ireland, but i imagine it must be the same in the UK. Log on to your provider's website and check. I know that both O2 and Vodaphone here give each of their customers 250 free web texts per month, and all you have to do is log on. You can set it to send to as many people as you like, and the website has the capability to hold the numbers with names, so that you just click on the recipient each time, rather than searching for their number. And you can set it to send at whatever time you like.
That said, if it's just a reminder to yourself, your phone has a free reminder function too...