Bikesafe courses are good but may well be booked up by now. They're also one day of training.
IAM courses are £140 for a year of training. They have a structured way of teaching and the end result is being examined by a police class one rider. ROSPA may well have similar prices but the difference is that instead of pass / fail with the IAM, you can get three grades of pass and you must be retested every so often to ensure you're still at the required standard.
There is a downside in that you may have to put up with the occasional arrogant "I'm better than you and because I do this for free it makes me amazing" arse and if you get a choice I'd go for the older guys - the ones who have been riding for 40 odd years, have seen it all and what they say is gospel.
The ERS is just stupid - it's pass plus for bikes and frankly is expensive and gives you little in comparison to similarly priced courses. I was told during my training for test that I'd tick all the boxes for the ERS and I just had to pay him to go out and do the assessment... Err.. if I tick all the boxes training for my test then what else is it giving me beyond test standard? Avoid.
There are also expensive advanced instructors but whether they're worth the money is seriously debatable - the IAM guys do a pretty good job, base their training on Motorcycle Roadcraft (I suggest buying a copy and applying it) which is the bike copper's textbook and have a decent assessment system.
Frankly, IAM or ROSPA are the best value for money and give damn good advanced training. Just go for which ever one is closer to you. Also, there will be people who look down on you for taking advanced training saying it's "boring". If you find advanced riding boring then fine - it's always up to you if you apply it but the odds are it'll save you the inevitable off or two that most people get post test. It will also massively accelerate your development of motorcycle control at this stage.
Insurance... I put advanced courses down and got nothing off. I think some insurers give 5% or for IAM and ROSPA as well as maybe ERS so but nothing to get excited about. The cost savings come in not having to patch up a bike and replace your kit.