Once you register for My Vodafone, you'll receive 600 free worldwide webtexts to send each month.
well yes, I'd imagine that you'd need to be a customer. But maybe you could get a ready to go SIM card, and top it up once every six months, and get 600 free webtexts a month.. I'm not sure if this would work, but if it does then that might be good for the OP. You wouldn't even really need a phone for the SIM card, just put it into your current phone to top up (if it's not SIM locked), then take it out and don't use it, just use the webtexts.....
[broken link removed] is a cheap website for sending web texts. I think it is 4c to the UK.
I have used their service up and down for both texts and calls.
O2's website lets you send 250 free webtexts to any mobile number. See here - [broken link removed]
EDIT: Sorry, just saw that you're already using that service. But that page also says additional texts are only 10c per text?