Its a bit like aviation in some ways. You have to pay for your training (normally) and you start with the lower paid jobs. Also the originally simple UK licences have been overruled by the EU so they all changed and now you have to do a rigid licence before you can train and take a test for artics.
I got an HGV class one (i.e drive any size lorry inc artic and trailer rigs) roughly ten years ago. This licence is now called C + E.
Like aviation you get clobbered for medicals, but because any quack can do an HGV or PSV medical, there is actual competition and although some charge nearly £60 I have just found a local quack who is doing them for £25. There is no ECG and the medical lasts until you are 40, then every 5 years. If you have a class 2 Aviation Medical you won't have any problem except for a few eyesight differences from the Aviation Medicals.
I already had a PSV class one (double-deck crash gearbox) licence prior to HGV training and it took me 5 days full time with the test on the 6th day. Total cost was about £500 ten years ago. Yellow pages ads will soon give you up to date info.
Job prospects: Here in the East Midlands work is always available through the agencies. If I wanted to I could pick up the phone and be driving tonight or tomorrow. If you have common sense and are reliable you can have as much work as you want. Pay here in East Midlands is £5-£7 per hour basic for agency work. Greatly increases for shifts, nights, overtime, Sundays etc.
Also consider PSV (now called PCV under EU rules). This is a lot more pleasant than lorry driving if you are a social creature. There are ways to get a PSV (sorry PCV) licence free of charge round here. You can even find bus companies who will train you free of charge in the evenings so you can keep your day job until your are qualified. The catch is that you have to work for them for a minimum time after you get your licence and the work is stage-carriage, not private hire. Stage-carriage pays more per hour than private hire, but less scope for casual work and you don't get tips, which can be considerable.
The top end jobs are hazpack trained drivers for the big petrol companies. I believe they are on £35k plus. At the other end, there are plenty of HGV drivers on only £300-£400 per week.
Why don't you ring some agencies in your area. They will tell you what they are looking for and what they pay.
One nice thing, once you have your licence, all you have to worry about is the occassional medical. There are no IRs to keep renewing. To sum up, not a get-rich-quick scheme, but a useful back-up to enable you to always be able to get work (and therefore money) quickly.
If you need more info, just ask me.