What does "back-haul" and "non LLU" mean?
In this context, back-haul is all the infrastructure between your ADSL modem and their L2TP server in Telehouse. So your phone line, the DSLAM in the exchange, the BRAS and the data network that links that to London. Most ISPs are in the same position here - they rent this stuff wholesale from BT, and it costs them to do so. AAISP pass on the install and cease costs, and notice period directly. The majority of ISPs absorb those costs and tie you into an n-month contract to recoup them.
LLU is a scheme whereby ISPs rent space to install their own equipment in your telephone exchange, and rent the bare wire to your house (and whatever fibre etc they need to get the data back to a central location). This is cheaper to rent, and eliminates the BT equipment (which is often a source of faults and arbitrary service restrictions), but obviously there's a cost associated with installing the equipment - hence you're only likely to have this option in bigger, more urban exchanges.
Practically, there is an administrative hassle (and cost) involved in transferring a line between BT wholesale and LLU equipment (someone has to go and physically plug things in). On the other hand, if you're using a BT Wholesale line with say, Plusnet and want to transfer to a BT Wholesale line with AAISP, the overhead is minimal, hence the token £1 migration charge.