I assumed she was not "allowed" to go home any other way. Because after work didn't she have to sit in the car with him for hours doing "surveillance"?
It was Sarah and John that needed to sit in the car and do surveillance with him, I don't think Sophie did? It was part of the MI5 cover (but that didn't exist for Sophie and Sandra). However, he did make Sophie wait and wait and wait for him to pick her up (just another form of control and way to isolate her).
She also said that she was paying £300 pcm board but only making just over £400, so it's quite possible she simply didn't have the money for a taxi and knew/suspected that if she'd got one home he wouldn't have paid it, or would have made her life hell for it because he was 'on his way, why didn't she wait rather than wasting money' or similar.
It's also quite possible there were no decent bus routes either. He had been living with them and manipulating them for a while but this point (she lived with him for two years I think, and this was the last straw for her) so it's quite possible he'd 'kindly helped' her find a job where she'd have to rely on him to get her there etc.
We didn't really get enough backstory to make it fully clear, but I can't imagine someone would choose to sleep on their work floor rather than going home, if getting home was a viable alternative.