//…the word 'restricted' gives me the impression that those major roads are not included.//
A "restricted" road is any road which has a system of street lighting, with lights placed no more than 200 yards apart. There is a slight deviation from that definition for "trunk" or "classified" roads which had lighting installed after 1957, but in general that's it.
The default speed limit on such roads is 30mph and no repeater signs are required for its enforcement (the street lighting denoting the restriction). Local Authorities have the power to alter the default speed limit (either up or down) by means of a Traffic Regulation Order. If they do, the new limit must be correctly signed. This does not apply to motorways.
I'm sure they've thought about it, but I'm not too sure how the Welsh Assembly are going to impose this. Restricted roads and the speed limits on them are defined by a UK Act of Parliament. I don't believe it will be good enough for the Welsh Assembly to simply say that in Wales, substitute 20 mph for 30mph in that legislation. As well as that, roads subject to a 20mph limit or those designated 20mph zones (the two are different) have to have signage (for 20mph limits) or a system of traffic calming (for 20mph zones).