I'm a mortgage broker, so I get to look over these mortgages quite closely.
IF's offset mortgage is as good as it gets, it's very flexible and easy to understand and YES a mile better than The One Account.
However, it is worth doing a comparison against the best 'regular' mortgage and seeing how much you'd need in the way of savings to make it worthwhile. Also, if your only ambition is to repay capital, almost all mortgages allow overpayments so again it's worth analysing which features you need and then looking for the best mortgage with those features.
By all means give me a call if you want advice, I don't charge.
Adrian Garside