I've been looking at systems such as txtlocal, esendex and clickatell. I need to send out a very large number of messages and ideally would like to go in at a lower level then using systems like these. Does anyone know how these SMS gateways like I've listed work in terms of actually sending out the messages? Will they have agreements with different carriers and be sending them out programmatically? I've tried contacting some UK carriers directly but as of yet haven't had any success getting any information from them.
Aggregators typically work by talking directly with a mobile carrier's internal SMSC using IP/X.25/frame relay and using a protocol like SMPP/CIMD to request a message send.
They will have connections to multiple networks SMSC's so they can do least cost routing (i.e sending a message to a user on their home network being cheaper).
Here are some contact details for Orange/Voda.
That said, MXTelecom as mentioned by Phill offer a good gateway service, as do mBlox - both of whom have already done all the hard (and expensive) work for you.