Ah bless you, its really hard when that happens. I too am under a lot of pressure to continue as normal at work and as alot of my day involves going to construction sites its not easy. At the end of the day your health is far more important for you and you LO than your job. Can you set up a virtual meeting with your maternity cover over conference calling/skype or some other method?
I mean what if you were driving all that way and have a dizzy spell and had an accident. it doesnt bear thinking about.
Speak to your mw and see if she can write a letter explaining the situation and that way when you do tell your boss its not because you say you cant do it, its because the medical professional says you cant.
I think you ought to tell them sooner rather than later so that they can make other arrangements. I know its much easier said then done, my boss is an arse most of the time and I dont take enough of a stand, but 100 miles driving is too much in later pregnancy and they should have already identified that in their risk assessment and should have thanked you for your kind offer but declined.
Alternatively, can they arrange transport for you? a taxi? The cost of which might focus their minds a little!!
xx
and get yourself better.
