I'm not aware of BT restricting any particular channels.
AFIAK Its more down to the modem element in ther outer which monitors the SNR level in each frequency bin. If a particular frequency of a particular channel is too low, then the modem can ignore that particular frequency and instead use another one with a stronger SNR. The modem will monitor each channel and select only those frequencies with a strong enough signal,
IIRC its the DMT standard which allows the modem to utilise frequency bins with a strong enough SNR.
Theres also something called bit swapping which is done "on the fly" and on several occasions using DMTtool Ive seen my own router performing "bit swapping" on certain channel if the SNR dips too low.
>> Sure I read somewhere that BE don't use certain 'troublesome' frequencies
Yeah - bit of a strange one - 2053kHz-2156kHz, AFAIK none of the other operators appear to block that frequency. At first Be waffled about it having something to do with maritime, but have since backed down on that and have said at some point in the future they may review.