Hi Homefish,
The way it works is that the Local Authority are the planning authority and they decide to grant permission or not. TII (formerly the NRA) are a consultee when the site is adjacent a National Road. So if you apply for planning, the LA will invite comments from TII but are not bound to act on them. TII normally respond saying that making a new entrance on a National Road is not a good idea. The council might decide to take that into account by adding some conditions to the planning, or by refusing. In my opinion, the best thing you can do is this.
1) Write to TII Safety section asking them for a map of the Network Safety Ranking for the location in question. This will categorise the road at your site into Very Good, Good, Bad, or Very Bad (I'm paraphrasing) . This will be useful for you to know how much of a challenge it will be.
2) Have a look at the planning website for your Local Authority and pick out a recent planning application on a National Road. Then read all the documents and see what happened in that case.
3) Armed will that information, go and have a pre planning meeting with your planning officer and just ask what they think of this issue (and any other issues while you are there).
You might decide to hire a local architect who is used to all this stuff and they will help you with all of the above if you wish. You can do it yourself but it will be slower.
Best of luck with it...