That still doesn't explain the need for RBS, HSBC, et al, to have branches located off-shore, and not just in Jersey.
I spent many years designing and coding software for a UK stockbroker with approx. 200,000 active customers. So, of course I needed to understand how each financial product works in fine detail, e.g. an ISA. They didn't have any products which could be described as sophisticated wealth generators, nor did they have an off-shore branch, but they do have highly competent staff.
Also, there's no shortage of private banks, and the like in the City of London, with staff just as competent as any Jersey bank staff. In fact, many, if not all, senior bankers and directors in Jersey, are not from Jersey, anyhow.
So, the question remains as to why these banks and services need to be located in Jersey, Guernsey, IOM, etc.
And, the only apparent jargon free answer I am aware of is the difference in taxation and secrecy.