@ScarboroughFairy , I mean this kindly, but - you are not safe to make this drive.
If you do 40 on a 60 single-carriageway, you will cause a massive tailback behind you, with the car immediately behind you able to see that the road ahead of you is clear and they will want to overtake you. This may cause some of those drivers to take risks just to get past you so that they can drive 'normally' and if the risk they take does not come off, how will you feel, knowing you are the root cause of the accident? That's if you can feel at all, because if someone crashes overtaking you, your car will almost certainly become involved on the crash.
If you do 40 on a 60 double-carriageway, cars will be able to overtake you in the second lane, but your car then becomes a pinchpoint, with everyone having to move into the second lane in order to pass you - you are effectively reducing the road to a single-carriageway. There's a reason to make a carriageway double, it's because there is a large amount of traffic using the road and it's going to be constant tailbacks and jams if it remains single.
The SAFEST speed to drive at is the same speed as every other car on the road. All cars travelling as one move smoothly, whereas cars moving at different speeds will introduce overtaking manoeuvres, which are inherently more dangerous that all cars travelling as one. So by YOU travelling at 40 when all others are at 60, YOU introduce the danger to yourself and others.
It may not be possible to book public transport at this short notice. In which case, either drive at the speed of the road, or not at all.
I would suggest you get yourself some motorway driving lessons for future. But for tomorrow - public, 60, or not at all.