pixiecake, I asked my consultant the exact same question (have elcs scheduled at just over 39 weeks, NHS hospital).
Her advice was - don't worry, if you go into labour before this date, just get yourself into hospital asap, explain your situation and you will get a CS, they just slot you in as soon as they can that day.
A midwife also told me to err on the side of caution - if you have pains/strange feelings, don't wait and wonder if you are in labour. Better to turn up, be seen, then realise it was constipation! and get sent home than to wait at home for ages waiting for it to get worse, and then turn up in well established labour. She did say this was the opposite advice that women aiming for a VB get - they are encouraged to stay at home as long as possible, and get on with things there. So I was told that if I thought I was in labour and rang the hospital, and then was told to have a warm bath or something, to ignore that! And just get myself in. (the same MW, who was very funny and down to earth, said that people sometimes try and 'scare' elec CS patients by telling them they won't be able to have one as planned if they go into labour beforehand. And to ignore them. How weird is that, though?)
She also said, the only time they can't perform a CS, and have to use low forceps/ventouse instead, is when the baby's head is so well descended that they'd damage it/you trying to 'push it back up'. The chances of you getting to that stage while still being in much doubt about whether you are in labour are pretty small...