I sometimes solder inaccessible T&E connections, although I don't know if you are using standard T&E.
The PVC insulation on 70C T&E PVC does seem to be significantly less able to withstand soldering than that on most connecting wires which you are likely to have soldered. If you solder it, make sure there are no forces on the PVC, e.g. dead straight into the connector or it will peel off the conductor as it attemps to straighten out, and nothing pressing against it as it will push through.