If you ride a MTB you should be carrying a small toolkit for self-help, this can be a life saver when out in the hills especially in winter. I carry a multi tool, a mini chain splitter, spare inner tube, tyre levers, puncture repair kit, a spare gear cable, spoke key, as well as a pump and in winter, a survival blanket. On an MTB breakages can and do happen; you can count on at least one "mechanical" on a group ride. The MTB code is that you're self-sufficient.
If you are road riding I guess your chain should be in good enough condition not to break. A cellphone might be your best option, or at least a £1 coin!