Welcome to the forum, Rich. I got a bike a few weeks ago - 30 years since I've been on a bike - so I know exactly what you mean about gaining your confidence. I am mostly scared of being hit by a car or scraping a parked car. I think a turbo trainer is a great way of getting your confidence. At the moment I am mostly riding on the canal path til I get more confident on the very busy main roads.
I am about 3st overweight. I am not expecting the cycling to help that much - 90% of weight loss is diet - but joining a gym has helped hugely with fitness. I am a member of The Gym group gyms and it is only £15 a month. Lots of heavy folk there, nobody cares, and weight training has really improved my fitness levels.
Cycling will be a great benefit to weight loss if you're motivated enough to stick at it. Of course you need to think seriously about your diet, but look at the two thing as two sides of the same coin, they shouldn't be mutually exclusive. I'm speaking from experience as I've lost over 3 stone myself, and it doesn't happen overnight - it took me 7 years overall to get back down to my 'fighting weight', but I'm here now and I have no intention of letting it slide. I enjoy the excercise now, honest, and you will too!
The best piece of advice I can offer is to ride as often as you can, eat a bit less and cut back on the bad stuff. The key thing IMHO is not to go down the 'hair shirt' route because you run the risk of quickly demotivating yourself and falling back into bad habits. The trick is to form new, better habits over time that you can sustain for the long term.
Best of luck etc...