Dear, Lyle!
As I know (from en.wikipedia.org/wiki/MTPAS), MTPAS differs from ACCOLC only by organisation procedures (On what level decision is taken) and more flexible assignment of access class on local level. MTPAS and ACCOLC are British internal procedures, based on standard GSM Access Class Control.
MS Access Class is stored on SIM and I am doubting that all British people want to re-programme their SIM (more so this hardly possible). And if however 1-2 malicious subscribers will preprogramm SIM, it not will cause a global network congestion.