Surely limiting a £50-60K BMW is stupid when a 19grand Vauxhall Astra can reach almost the same top speed....?

It's an agreed speed decided among manufacturers, BMW, Mercedes & Audi especially, to cope with generally available tyre technology.

As already said, it is because of a gentlemans agreement between Audi, BMW and Mercedes.

The coming about of the agreement is nothing to do with tyre ratings though. It was because the German government were threatening to put speed limits on the unrestricted sections of the Autobahn. This was because cars were getting faster and faster.

So the big 3 in Germany made a deal saying that they'd restrict their cars if the government backed down. Deal done.

Since then, other manufacturers have followed suit but given their reason as safety. Some manufacturers didn't bother (most notably Porsche) whereas others put a different limit on their cars.

It's either Subaru or Mitsibushi that set a 160mph limit, so that their cars were faster than M5s etc.

The agreement has recently been dropped although I suspect manufacturers will maintain it on safety grounds. Audi are releasing the R8 with no limiter on it as a direct challenge against the Porsche 911.

Mercedes tuning arm AMG will raise the limit to approx 185mph if you ask them, but will not go higher because they will not supply tyres rated high enough.

Other tuners will remove the limiter completely allowing a BMW M5/M6 for example to hit over 200mph. Tyres are available that can handle that sort of speed, but they are not "approved" by the car manufacturer.

Your £19K Astra could indeed hit 155mph, but it certainly wouldn't do it as comfortably or as fast as a £60K BMW/Audi/Merc.