Depends where you live, but I trained at Aga/Rayburn where you learn to do vaporising appliances too (e.g. Agas, Rayburns, oil stoves, etc.) There's something really old fashioned about these contraptions as they're basically a pot of oil that quietly burns the oil.
You'll probably need the OFTEC 50, which is an introduction course (one week). Then you do your 101, 102 and one or two others if you want which takes another week. Then it's around £800 for your OFTEC registration for 5 years. An inspection in your first few weeks/months. This isn't really a test, but much more of a session that you can ask loads of questions and talk to your assessor who is very approachable. And that's about it. It's also sensible to go on a day's course at a manufacturer or two where you learn more about the boilers and maintenance and less about the red tape.
No NVQ required. No buddy work required. No nasties like slapping DO NOT USE stickers all over the place and ruining someone's life (very rare something's so dangerous this happens and if it does you're expected to fix it before you leave.) (We're a friendly bunch us OFTEC people.)
I don't do gas as I live in a rural area and I have around one oil job every couple of weeks or so in the winter. Oil boilers are pretty sturdy things and realiable. Maddening when they go wrong but most satisfying to fix.
Give Aga (training) a phone call and one or two manufacturers (e.g. Worcester, Grant, Mistrel) as well to compare notes and costs, etc.