The BT checker gives an estimate (and a very conservative one, at that).
In reality it seems nothing more than a wild guess.
I get this result (telling me what speed I can get): -
Our test also indicates that your line currently supports an estimated ADSL2+ broadband line speed of 2Mbps; typically the line speed would range between 1Mbps and 3.5Mbps
My line speed is actually around 4.7Mbps (give or take 0.3Mbps) - so way more than BT's stab in the dark guess of 2Mbps.
My data speed is normally at least twice the BT's guess for the line speed alone.
If you're currently getting 6Mbps, that's what you'll get with BT.
Unless.....You're currently getting that speed because you've tweaked your noise margin down (which tends to upset BT's systems, resulting in a higher margin).
If you're not on an unbundled product (e.g. Sky, Talk-Talk, BE, O2) and are already on a BT Wholesale connection, your connection speed will remain the same (as all that will happen is BT will direct your data-stream through their own networks, rather than your current ISP.
ADSL2+ with BE
DL Sync around 4.8Mbps
UL Sync 1088kbps
DG834GT with DGTeam firmware