Interesting post Richard. I probably fit into the rider/builder category. My bike is far from shiny and has the scrapes of it's past etched into it's paintwork, but I keep it mechanically sound, and have made sure that all the components are true to it's build year.
I bought it because after a 15 year layoff concentrating on riding motorcycles I missed the fitness and waistline I had when I was doing 500 miles a week round London as a cycle courier in my late twenties
Don't know what it's like in Northern Ireland but here in Sheffield traffic conditions are pretty similar to London in the early 80's, and the hills mean that I couldn't get by on your bike's gearing.
Dealing with traffic, and trying to get familiar with riding and your leg muscles back is a big ask. One thing at a time, and I think the idea of getting a cheap mountain bike and doing some off road/trail riding so that the bike becomes instinctive is a really good idea. Trainers are not a good idea, the static nature does not give you a feel for the bike, or the reaction times needed for road riding.
Once you have done this, you will find that a decent rider can out-accelerate most traffic for the essential 10 yards it takes to get stable from a standstill. Other than that, the key to safe riding, as NeilM pointed out, is the same for a bike as it is for a motorbike. 100% awareness of whats going on around you, and a focal point of about 10seconds ahead should enable you to be prepared for what is coming up. So many riders and drivers focus on the car ahead. this is too close, and will not enable you to position yourself for the approaching sideroad turner/people exiting cars/petrol stations etc. Make sure you move the bike from side to side, it is harder to see a bike travelling in a straight line than one weaving. Position yourself so that cars can't squeeze you on narrow roads, and consider using daytime lights. Oh, and keep momentum up on the straight. Drivers are less likely to be aggressive if you are pedaling hard, if you are coasting and holding them up they get wound up.
Hope this is of some help.