The problem with LEI is that your insurer will inevitably pass your case onto one of its panel solicitors which is fine if your case is simple one; e.g. a straightforward employment issue, but if the case, or the need for advice, is out of the ordinary or complex (an AAIB investigation springs to mind), you will struggle to find a solicitor with the necessary experience. Even if you know of a firm with specialist knowledge and expertise (such as Stewarts), unless they are a 'panel firm', in my experience most LEI insurers will normally refuse to fund the necessary advice: i.e. go 'off-panel'
In the old days unions were much more flexible but its so long since I had any dealings with unions, I can't say what the current position is.