While you're reading this there's a good chance that a few GA aircraft in the UK are flying with incorrect altimeter settings - they departed the circuit and forgot to set QNH or they rejoined and are flying the pattern height at altitude instead of height because they haven't set QFE. Every time an altimeter sub-scale setting is changed there's also the chance it'll be mis-set.
The UK altimeter setting protocol and transition altitudes were formulated many decades ago. Regional settings were intended to provide an approximate QNH when it was not so easy to get reliable updates of local QNH. The climb performance of the average airliner meant that several minutes would elapse after take-off before TA was reached. Nowadays there are plenty of en route sources offering current QNH and many airliners, especially twins, can reach TA much more quickly. As another poster has mentioned, flying on regional settings might lead to airspace infringements.
Every time a low pressure weather system crosses the UK the CAA have to issue panicky notams reminding pilots of the vital importance of altimeter resetting at TA.
The Americans have got it right - just one setting for altimeters below transition altitude. I understand that - at long last - the UK CAA (and EASA?) are consulting on raising TA to 18,000 feet. Long overdue!
More on the topic of altimeter setting
here.