I know LLU is third party broadband providers with their own equipment installed at the exchange.
I know that FTTC is fibre from the exchange to the cabinet and wire from the cabinet to the home.
Yes, but still retaining the wire from the cabinet back to the exchange, to provide the voice side of the circuit.
I think that 21CN is an upgrade to the existing BT wire network.
No, it's a change to the DSLAM equipment in the exchange. Meaning that BT Wholsale based ISP's can provide an 'up to 21meg based service rather than the existing 'up to 8' service. If you already have LLU in your exchange, they will already be providing an up to 21 service from their kit, so a move to LLU would hopefully mean a rise in sync rate... This dependent on line length, etc.
FTTC will only be used for BT products? Or will non BT/non LLU ISPs be allowed to use the FTTC network?
No, it's actually a BT Openreach product (slight difference), have a look at Bob's
info on the subject.
I'm about 600 metres from my cabinet. Am I right in thinking that I could get in the region of 20mbps with FTTC?
No, I'd suspect nearer, or maybe even the full 40meg.