Greetings OhNoCB,
To your first question, I would expect this to be almost invariably what the FISO would say in response to your routing request. There is little else s/he can say within the legal framework for providing a basic service. "Your discretion" indicates that the responsibility is yours to remain clear of terrain, danger areas, CAS and so forth on your way to destination. If however, on first contact with London FIR, you state an intention "to rejoin CAS at XXXXX for Biggin Hill" then I would expect the FISO to facilitate that for you with the sector responsible for that CAS, but how you arrive at that joining point remains your own issue.
With respect to your being an IFR flight handed over to London Information, I would suggest Dublin have little choice, as they're unlikely to have direct telephone lines to the other agencies who might be interested in working your flight outside CAS (eg Western Radar, London Mil). Again, it is up to you to ask on first contact if you feel a radar service would better suit your needs. And again, the FISO will more than likely try very hard to accommodate this request with the most appropriate agency.
I'm not familiar with the airspace in your final example, but I would suggest that if a sector controller is not too busy with traffic inside CAS and s/he feels they are able to provide you with some level of service outside, s/he will offer such. Again, there may be limitations (for instance, where I work we are not permitted to provide a radar service below FL70).
LTP