There is also the issue of "
Unexpected Necessity", which relates to those situations where you end up taking a taxi, cab or friend giving a lift and you don't have the necessary child seats etc.
The emphasis is on unexpected, so cannot be used as an excuse for any journey that, reasonably, could be considered pre-planned.
In most cases, these state that children under 3 may travel in the rear seat unrestrained, whilst those over 3 may use adult seat belts but still travel in the rear.
The only exception that relates to front seats in any way is;
"Where two occupied child restraints in the rear seat prevent you fitting a third. In this case, provided the front seat is occupied, a third child aged 3 years and over can use an adult seat belt (lap OR lap and diagonal) in the rear. If the front seat is free, then they must seat there using the correct child restraint."
taken from http://www.dft.gov.uk/think/focusareas/childre n/childincar?page=Overview