Because of this little spat I thought I'd actually check out what the standards are and do a bit of background reading.
According to BSI
Government guidance (which makes reference to Road Vehicles Lighting Regulations, specifically
SI 2005 No 2559) refers to BS6102 part 3 or the "corresponding standard of any EU country".
BS6102-3:1986 Cycles - Specification for photometric and physical requirements of lighting equipment is current but the BSI charge £158 for a download.
The alternative is any EU equivalent, and anecdotally Germany springs to mind when it comes to bike lighting standards. Thats Straßenverkehrs-Zulassungs-Ordnung or
StVZO (I hope your German is good)
Some manufacturers will get products compliance tested to StVZO if they are aiming at the German market. No-one seems to give a damn about BS6102-3 because ... well no-one else gives a damn, so why should they? If you are a compliance-obsessive, go for StVZO compliant lights. Then you get some kind of guarantee that your lights will be safe and effective. Which isn't to say that non-compliant lights are
necessarily unsafe or ineffective, you just don't know.
The chances that not using a compliant light will cause you any legal problem are very very low. The chances that you buy a non-compliant light that dazzles other road users, confuses them, or is otherwise ineffective or counter-effective are real.
Conclusion: It's a complete mess. Nobody enforces it. Manufacturers don't give a damn and don't get their equipment compliance tested unless they are aiming for the German market, in which case they get StVZO certified.
Oh yeah, and pedal reflectors are mandatory.