The LLU scenario to which the BT representative is referring is the equipment installed by your ISP (along with many others) to handle your broadband and, if part of the package, your phone calls; it bypasses the BT equipment.
From the exchange, the normal BT line to your home takes over.
An LLU equipped ISP can provide much cheaper services for phone and broadband that BT, which is desperately trying to win back customers with new offers.
However, if you study BT's offers, you will realise that they fall far short of ISPs such as TalkTalk, yet cost the same or even more.
The BT rep is clearly trying to bamboozle you with fancy words and is being, to put it kindly, naughty.
Re changing from an ISP who has LLU equipment in your exchange, steps were taken a year or more ago to make the process much more simple and straightforward.