Why leave London for the sake of leaving London? The UK is a small country and is very much centred on London. London has the best connections to other places within the UK and is the most connected place on an international level (re. flights). The Greater London Area contains almost a quarter of the population of the UK.
Most UK cities other than London would require train journeys or internal flights within the UK for those flying into the country.
Change for change sake isn't necessarily a bad thing when it comes to making city bid like this.
Bidders get tired of submitting the same city again and again. Juries get tired of reviewing the same bid again and again. After a few times, whatever reasons they didn't select a city the last (few) time(s) is going to dominate over actual bid this time around.
Disclaimer: Not saying that necessarily applies here (yet)
KTC