Possibly would be seen as tax avoidance however unless you are a current 40% earner or director so have to fill in a tax return I dont see anyone ever looking into it.
The interest earned is still taxable in the UK even if it isnt brought into the UK if you are UK resident.
Inland Revenue Forign Income
I think also soe offshore investment funds you pay capital gains tax rather than income tax but this may have changed.
You may be better looking at some of the tax free savings options withing the UK and filling these up first.
As an IT contractor I may have to work abroad at times and it is a bit of a minefield especially places like Ireland and Switzerland.