I can hazard a guess at a few answers.
1. Certinally in terms of ad-hoc flights we file flight plans all the time for pilots through are AFTN and this costs them nothing. Although i suppose they are paying us for our air traffic control service. However these ad-hoc flights are not being billed by us anyway.
2. Operators are not obliged to follow airways by any legal boundry that im aware of. Obviously the operators would prefer their pilots to stay in nice safe CAS rather than dropping out into bandit country so to speak to know a few minutes off the flight time.
3. If your filing IFR on your flight plan your tied to using the correct routings. The systems are automatic most of the time. If you file IFR on a route you WANT to take but Eurocontrol say that route isn't a recognised routing from ABC-XYZ then they will give you the correct routing and like it or lump it. If everyone did everything they want it would be manic so its regulated. If they don't like the routing they will just give you the one they do like. This applies to your Oxford to Bournemouth flight.
Your other options are to file Y/Z on the plan and go IFR to VFR of vice versa but this is dangerous, if you are on route and meet some weather and say eek i wana stay IFR in these lovely airways they will most of the time chuck you out at your intended VFR point. So if i were you id leave this one alone.
You can always file IFR and change en-route so get your airways clearance go up fly it for however long you want then say i wana drop out of CAS and take up a radar service from Farnborough or whatever...
We 'organise' IFR cross-country hops for flights all the time. Alot of our flights want to reposition from us to east mids or like with the Crossair's they like to go off to the west through military to Ireland. They arn't very easy to organise the first time but once the units have seen the same flight doing the same thing an unwritten procedure falls into place. So if your doing a regular hop from Oxford to Bournemouth, phone the relevant units en-route and see if you can get them to co-ordinate a route for you, more time involved but once you've done it it becomes sort of standard.