The difference between LLC-mux and VC-mux comes about when multiple protocols are deployed over the same ADSL connection. LLC-mux allows protocols to be separated on a single ATM virtual circuit, whilst VC-mux requires one ATM virtual circuit per protocol but has lower overheads. As ISPs on some networks are charged per virtual circuit, they sometimes use a single virtual circuit with LLC-mux to keep costs down.
ADSL over BT Wholesale (or over the Zen network if you are on an exchange where Zen has presence) is a single protocol product, so there's no advantage using the higher overhead LLC-mux. The suggestion is that you stick with VC-mux unless you encounter problems using it. Whether you'd notice the small extra overhead of LLC-mux is debatable, but why add unnecessary overhead?
If you use the recommended PPPoA, your connection should work with an MTU of 1500.
If you use PPPoE, your connection is limited to an MTU of 1492 because of the extra 8 byte per packet overhead. There is a way of using PPPoE with a 1500 byte MTU, but it is not supported on either of the BT Wholesale ADSL platforms.
In summary, PPPoA VC-mux imposes the lowest overheads and allows a full 1500 byte MTU with a suitable router.