From messing around on a spreadsheet - hopefully someone will check.
Ignoring fees, you'd be better of paying 41% tax over ~5+ years if it's taxed at exit or deemed disposal if the interest rate is just 0.1% higher than the same savings taxed annually by DIRT@37%.
(That'd be assuming the DIRT is deducted from the account annually - reducing the compounding - and it's 33%+4% which won't always be the case)