Have found clear legal references on this issue.
Flashing your brake lights is legal during an emergency stop under clearly-defined conditions only.
Regulation 48 ***8211; Lighting Installation: Amendment 4 to Revision 4 of ECE which entered into force in UK on 12th June 2007 stipulates the requirements for emergency braking indication (ECE/Trans/505/Rev.1/Add.47/Rev.4/Amend.4).
"Emergency stop signal" means a signal to indicate to other road users to the rear of the vehicle that a high retardation force has been applied to the vehicle relative to the prevailing road conditions."
"The signal shall only activate under one or more of the following conditions:
- the vehicle speed is higher than  km/h and the deceleration of the vehicle exceeds  m/s2. The speed of 50 km/h has been selected to ensure that the ESS should not be activated in urban, low speed environments, where high decelerations may be readily achieved;
- [the physical limit of tyre adhesion to the road surface has been achieved.]. This is the case when the ABS is activated. No speed criterion is included in this instance, as the conditions which might usefully warn following road users would include low speeds and deceleration occasions when there is a low coefficient of friction between the road and tyre (e.g. icy conditions)."
"The emergency stop signal shall be activated only when the vehicle speed is above 50 km/h and the braking system is providing the emergency braking logic signal defined in Regulations Nos. 13 and 13-H."
"The emergency stop signal shall be automatically deactivated if the emergency braking logic signal as defined in Regulations Nos. 13 and 13-H is no longer provided or if the hazard warning signal is activated."
"All the lamps of the emergency stop signal shall flash in phase at a frequency of 4.0 ± 1.0 Hz."
"However, if any of the lamps of the emergency stop signal to the rear of the vehicle use filament light sources the frequency shall be 4.0 +0.0/-1.0 Hz."
"The emergency stop signal shall operate independently of other lamps"
"The emergency stop signal shall be activated and deactivated automatically."