Arch,
First of all, welcome to the forum. Though I'm in the U.S., I'm sure it would be interesting to our U.K. members to have a general idea of your whereabouts and, maybe, useful to you to know fellow enthusiasts nearby.
I can't say what the MOT or insurance regulations require in the UK, but if it were my bike here in the U.S., I would choose my model moniker based on the vehicle ID used on the title. For example, if the bike is titled by the frame number and has half-width hubs I'd call it a Road Rocket. If it's titled by the engine number and you have full-width hubs, I'd call it a Super Rocket. If it's titled by the engine and has half-width hubs, I guess it's choose between Road Rocket and Rocket Gold Star. In this case, I'd be inclined to use "Road Rocket" so that I was not seen as illegitimately trying to make my bike appear to be more than it is. Really, right now it's not anything specific, so call it what you want, if the law allows.
Contrary to Tomcat's post, the "A7" in the '55 frame number does not designate the bike as an A7. Or, maybe he just believes that is how the insurance nerds would see it.
By the way, there may/will be dissenting opinions regarding my comments, and that's great, because I could be dead wrong. I look forward to hearing them.
Richard L.