Essentially, if you're looking for advice in relation to a dispute that is before, or may come before, a court or tribunal, you'll need a solicitor.
But in relation to pure advisory work, where there is no dispute, you can deal directly with a barrister. E.g. — How can I structure this transaction so that it will engage the tax treatment I desire? Can you review this merger/acquisition agreement and advise if it gives me adequate protection against risks X, Y and Z? Here is an unregistered title — is it a good marketable title, or should I choose not to buy this property unless the vendor first registers the title? I want to set up a trust to run a charitable home for elderly and infirm cats, funded into the future by a business that restrings tennis racquets — can you draft a suitable trust deed, please?
In each case I haven't yet undertaken the transaction; I haven't signed the merger/acquisition agreement; I haven't signed a contract to purchse the house; etc. I'm free to walk away and, if I do, there's no dispute, because nothing has happened. Hence, this is non-contentious business; it won't end up in the courts.
But if, say, I've already completed a transaction and now want to argue that it attracts this (low) amount of tax rather than that (high) amount, or I've already signed a contract but think I might want to get out of it, any advice given there is advice in relation to at least a contemplated dispute.