One should never file Y or Z in the UK.
IMHO, one should never file Y or Z in any other country either, unless CFMU actually chucks out the flight plan (which it does for some countries) with the "airport not having IFR procedures" message.
The problem with filing an IFR flight with any VFR section in it is that ATC are entitled (during the VFR section) to treat you as VFR traffic, and just tell you to report at such and such VRP, or wash their hands of you totally. So you need to be prepared, with current VFR charts and airport VFR plates, a VFR GPS, etc. And of course the length of the VFR segment is up to them - regardless of which VFR-IFR transition you filed on the flight plan.