Are people going to be texting into your number for autoresponse or entering their mobile on a website (your first sentence implies the former, the second the later)?
If you don't mind using a 10-digit number to send (over a shortcode of 5/6 digits), you'll get better prices than any of the straight bulk sms providers by creating a simple app using Twilio. They charge less than a penny a message outbound and a basic autoresponder script without anything fancy can be coded up in less than an hour by someone who knows their api. Lots of people on the forum rave about twilio so you should definitely be able to find someone to do the job in BST for a reasonable price. Its also pay as you go so no monthly minimums or commitments.