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Best way to make most of savings with ISA and Offset mortgage
I am not a Financial Advisor, but I an tell you what I did in exactly this situation - which is pretty much what you are proposing.  I put money into the offset savings account until I had only a small amount of mortgage "balance" left (less than a year's worth of mortgage payments), then I set it up so that each month I did the transfer from the offset savings pot into the mortgage itself. This depleted the offset savings in line with the mortgage debt, and the interest on the two balanced out almost to zero. This was self-sustaining and meant that I kept the same margin owing over time (i.e. if I was in this situation for 5 years, for the whole 5 years I would effectively have 1 year remaining on the mortgage). Meanwhile, since I now didn't have any mortgage outgoings from my regular income, I put any spare money into ISA savings. No need to withdraw money from the mortgage to move to the ISA. The benefits of this (as opposed to just paying off the damn mortgage already) were that I kept the full liquidity of the mortgage amount - I could withdraw all the offset savings pot if I wanted to, although I would then have to have funded the mortgage payments differently, and as that liquidity went down over time I was building up other savings in parallel. It worked well for me. It almost doesn't matter what the offset mortgage rate is since you are effectively paying it off by keeping the offset savings pot so high.