There are a few ISPs out there that provide the ADSL bonding, Andrews & Arnold are another.
Agree with SJ’s comments above, the advantage of EFM is that it will give you symmetric speeds so your upload will be the same as your download. ADSL even bonded isn’t going to match the upload speed you’d get on a single FTTC (what’s being marketed as fibre broadband) connection.
All comes down to cost, and your requirements for upload/vs download.
Also perhaps putting some sort of Acceptable Usage Policy in place, either in writing or technology (or both) so that you can clamp down on heavy internet hitters if you do end up with a limited bandwidth solution.