I am sure you can turn from a 70 mph road in to a 30mph road.
but in the road marking and signing manual which is A2 size and about 2" thick every combination and lay out of signing and spacing is laid out in the smallest detail. I have never seen a 70mph duel carraigeway drop directly from 70 to 30 Which sugests it may not comply with laid down regulations.
Remember that awfull Trevor Mcdonald program with all the combinations of signs and white lines? The same applies to motorways and duel carriageways.
I will dig the book out at work tomorrow!
Edit: Sorry my colleages no longer have the book..

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Speed limit sign radio interview. TV
Snap Unhappy“It has never been the rule in this country – I hope it never will be - that suspected criminal offences must automatically be the subject of prosecution” He added that there should be a prosecution: “wherever it appears that the offence or the circumstances of its commission is or are of such a character that a prosecution in respect thereof is required in the public interest”
This approach has been endorsed by Attorney General ever since 1951. CPS Code