I've lived part time in St Andrews and London as well as the US. First is that respect for parents is noticably higher in the UK (and Europe) than in the US and that is a good thing. OTOH, parents in the UK tend to treat their offspring as more adult, give them responsibilities at an earlier age, allow them to fail, and are less judgmental. Kids in the UK also don't sneak around to drink like US kids as they usually drink, somewhat responsibly, with their parents instead. Overall, parents and offspring in the UK have much better lifelong relationships than those in the US. Kids in the UK are much more likely to try to understand their parents concerns and negotiate with them than to say F U, I'm doing whatever I want.

Road fatalities in the UK are, like most of Europe, fairly low, about 1/4 that of the U.S. per capita. Bicycling fatalities in the UK are quite high though and relative to vehicle fatalities extremely high — and these are reported in the papers and TV. Bicycle fatalities are much higher in the US but so are road fatalities in general so bicycle fatalities don't stick out as much in the US as in the UK. To a parent in the UK bicycling appears massively more dangerous than it does to a parent in the US.

Many of the suggestions above about figuring out routes and educating your mum about how relatively safe you'll be are good and hopefully will result in her support. I would not make an enemy of her over this though — it's not worth it.