We too utilise email for the vast majority of our comms to our learners, but the only drawback is that we operate a 5 x shift rota. This means that if an event is cancelled whilst the learner is 'off shift' that an email is insufficient due to the fact that they are not in work to receive it!
Hence the need for a simple, automated comms tool to reach them whilst they are at home.
A supplier which is getting talked about in Yorkshire is http://www.smilessms.com/ I've not used them myself but they got good press round here when we had bad problems with snow causing school closures. Best regards Glynn Jung
I would thoroughly recommend using xlearn for SMS services as it is very reasonably priced (£25 for an account) and allows you to create bulk texts as you describe. It is also a super way to use as a response tool in the classroom (or out of the classroom) as the learners can reply back using a prefix word.
Lilian Soon of xlearn.co.uk can give you even more advice and ways to use SMS with learners can be found at LSN MoleTECH website.