Thanks for your replies, dreams_and_discoveries and Playing with Fire UK.
Yes, it would seem that there is no easy solution to the balance between saving in an ISA/taxable accounts vs a pension, for those in the UK who aim to retire significantly before the normal retirement age.
An ISA/taxable accounts have the advantage of accessibility, whereas a pension cannot be accessed until 55 (a figure which is subject to change). However, an ISA/taxable accounts do not carry the same tax benefits as a pension, especially for higher rate taxpayers.
I would be interested to hear how others have dealt with this issue, as it must crop up a lot for UK Mustachians. As you point out, Playing with Fire UK, the balance may also be affected by the decision to save for property purchase(s), which may steer one more towards accessible forms of saving.