Probably was limited to 155mph.
Another 'urban myth' that used to circulate years ago that Police Cars were 'souped up' was not true. They had additional equipment such as bigger alternators, firmer suspension and additional speedometers etc etc, but they weren't 'souped up' as such, and years ago were straight off the production line with local Police vehicle workshops adding the additional equipment relative to their use. In the 'good old days', they were possibly in a better state of 'tune' than Joe Publics like for like cars, which may have contributed to the thought that they were faster
Today, most manufacturers supply Police spec'd vehicles already equipped with the additional wiring looms and light bars etc etc rather than standard vehicles being equipped by the Police themselves. I often see transporters up and down from Ford for example with brand cars kitted out for the Police even with the livery (which is now to a standard - 3M for one) sometimes minus local crests. Some radio equipment is still unique to each individual Force, but the cars themselves are standard vehicles as supplied to anyone.
But every man on the street still believe Police cars are faster (which is no bad thing)