No undertaking is not an unwritten rule.
Not coming close to horses is clearly in the Highway Code.
You clearly need both etiquette and knowledge of how to ride.
Couldn't find any no undertaking rull for cyclists in the highway code. There's an advice for motorists tho to look out for cyclists because they can filter stationary or slow moving traffic from both sides
You disobey Highway Code 185 and 167 when you did that manoverve at the roundabout.
@Arturikus started
London Fixed Gear and Single-Speed is a community of predominantly fixed gear and single-speed cyclists in and around London, UK.
This site is supported almost exclusively by donations. Please consider donating a small amount regularly.
No undertaking is not an unwritten rule.
Not coming close to horses is clearly in the Highway Code.
You clearly need both etiquette and knowledge of how to ride.