Good news: with the latest update of the Tomtom GO software (15.600, published 14 january 2016), the tethering now works!
In fact, the GPS lost all its paired devices in the update, which prompted me to give my FP1U/1.8.7 another try. There was a bit of fiddling for pairing, and another few misses when getting the PAN going, but then, the famous “Do you allow the connection?” dialog could be brought up from the notifications, duly answered “yes, allow”, and then, lo and behold,
the connection worked.
In use it proves stable. A few hassles remain: the bluetooth network sharing function (in the “more>connection sharing” sub-menu in the settings) tends not to stick, and above all, this “Allow connection” notification shows up and has to be validated on each re-connection (if you miss the first one, which seems to get lost when unlocking the phone, hang on, you will get prompted again less than a minute later).
The FP is the ONLY Android or iOS device I know that systematically asks to allow network access for known (paired) BT devices. If this nag screen could be removed permanently it would be much simpler.
Good luck,
Jan