Ask HN: What's the best SMS API provider for a uk startup

I have a great idea in the SMS market but I can't tell which provider is best a startup just trying to get away with paying as little as possible. options are: (a) https://www.tropo.com/ (b) http://www.fastsms.co.uk/topic-pages/bulk-sms.html?source=aw-bulksms-2; (c) http://www.esendex.co.uk/; or (d) http://www.textmarketer.co.uk/index.htm. Maybe you know a better one?