http://www.sendsmsnow.com/
http://www.swagsms.com/
http://www.boltblue.co.uk/
http://www.cbfsms.com/
http://www.freebiesms.co.uk/
No idea how well and how quick they work! For true reliability, the UK o2 (
www.o2.co.uk) site also allows 10 free SMS's per month. You will need to register any UK number (doesn't need to be o2)
The first one mentioned (smslisto) is a Betamax/Finarea offspring. These are also generally reliable in my experience. Messages go through fast. However, with most of their brands the free SMS's are limited. You may need to top up and then will get a very generous of free or very cheap messages and can even set caller ID!