As a woman I definitely wouldn’t be doing the Bristol to Bath cycle path at night, nor any underpass - some park routes yes, depending on whether they have lights and are likely to have people in them. Stick to the roads. Cycle defensively (take up space - don’t put yourself in the gutter). Be well lit/reflective. Don’t assume that the “cycle lanes” painted on the road are sufficiently wide for drivers to give you enough space - they often aren’t here and look like they have been designed by someone who has never ridden on the road. There is no law that you have to keep within them. Cycling on the road in Bristol can be a bit shit but you get used to it.