Has anyone used an offset mortgage? Worth the higher rate?

My broker mentioned offset mortgages and I'm intrigued. We have about £40k in savings that we want to keep accessible (emergency fund + home improvement pot). With an offset mortgage that £40k would reduce our mortgage interest.

But the rates seem about 0.3-0.5% higher than standard products. Is it actually worth it mathematically? Or are we better off with a standard mortgage and keeping the savings in a high-interest account?

Anyone actually using one?