The nectar conversion puts a floor at 0.8p, so this is the absolute minimum value you can get from them if you cannot find any other redemption that suits you.

Typically I will not redeem them unless I am getting at least 1p of value. However it is easily possible to get 2-5p+ of value on certain J/F redemptions, especially if the full trip is outside of the UK and not on BA to avoid the obscene charges on reward bookings. I have previously booked long haul J flights out of the US on AA or out of Japan on JAL with less than GBP20 of fees/taxes, compared to the probably GBP500+ you would pay with BA for an equivalent flight out of LHR.