Nathan - the 2018 regs should cover most things, as they haven't changed significantly. Where there were changes, these are noted in the 2018 document along with the dates they came into force.
Regarding rollover bars, the current UK requirement is for either (a) certification or (b) dimensions for tubing that have to be used. Motorsport UK (formerly RACMSA) has published a guide here:
https://www.motorsportuk.org/wp-cont...8/ssrops-1.pdf
Hope that helps!
I'm guessing you are related to Martin Down?