Conservative investments are no good for mortgage repayments

Unfortunately, the safest investments are clearly unsuitable to repay a mortgage since the return is more or less guaranteed to under perform to mortgage interest. 

To recap, the criteria for an effective interest-only mortgage is that the net return from the investment must exceed the gross mortgage interest over the full term.  For example, to match a 7% pa average long term mortgage rate, you need a gross investment return of at least 8.75% pa to cover basic rate tax before any charges are considered.  In practise, you probably want at least a 1% pa better return than the mortgage rate to make the effort and risk worthwhile, so you are most likely looking at 10% per annum minimum gross return for a taxed fund after charges, or 8% pa for a tax free fund.  Where are you likely to find an investment to achieve that with any reliability?  

Put another way, is it worth the risk just to achieve a third of a percent reduction in your overall effective mortgage rate?

Cautious people choose repayment mortgages
The answer depends on your attitude to risk.  If you are risk adverse then there is no point in choosing a safe investment alongside an interest-only mortgage since it will never work out. So for security conscious people, a capital repayment mortgage is the safest of all since there is no investment to risk.