You can make this journey without going via London. However, the route is tortuous and involves at least four changes. You would have to travel from Liverpool to Wolverhampton (or Birmingham or Coventry). From there to Leamington Spa. Then another train to Princes Risborough. A Chiltern Railway service from there to Aylesbury and finally a London bound Chiltern Railway service train from Aylesbury to Wendover. The problem with Wendover (which is on the Aylesbury Line out from London) is that there is no service northwards beyond Aylesbury so you cannot get directly there from the north. Making the journey this way will take a good bit over four hours.
If you travel from Liverpool to London Euston, then scoot round to Marylebone and from there out to Wendover on Chiltern Railways, it will probably take you a little less (Liverpool to London is about two and a quarter hours, Marylebone to Wendover less than one hour, and about fifteen minutes changing in London). However, this way involves only two trains (plus a short tube or bus journey in London) and the likelihood of delay, I would say, is somewhat less.