If you mean on Plusnet, see
Section 2.
Your current gateway.
If you mean WBC handover nodes, such as the locations shown in Section 1, there are 20 dotted around the country. WBC-using ISPs use all twenty, WBMC-using ISPs (Plusnet and nearly all the other BT Wholesale ones) use a subset of those. Small ISPs may only use 1.
There are a separate 10 nodes for IPStream Connect, though they are physically at the same place as ten of the WBC ones
The handover at these nodes is via MSILs.