No idea how things are done in the UK or Europe, but in Canada, unless something has changed radically, air files, both VFR and IFR are relatively common, due to the extreme size of sparsely populated areas.
I have often been at locations where the forecast weather was VFR for the time I was to be there, but the only way to get weather reporting was when we were in the air. There were plenty of times we expected good weather heading back to civilization but in the north that can change quickly. Usually we would depart VFR (sometimes not, don’t try that anywhere except class G) and when we could make contact with center we would file as required to home. More than once we needed to transition to IFR to get through squall lines or to climb above unexpected freezing precip.
In general all the same information would need to be communicated as when you filed at flight services. I always kept a copy of the plan form in my flight bag so you wouldn’t miss anything when air filing. If in class G airspace things are pretty simple but you need to give Center enough notice to get your IFR clearances into controlled airspace unless you want to find yourself holding over a VOR somewhere.
ATC in Canada is generally pretty accommodating but I wouldn’t make a habit of throwing extra workload at them approaching the YVR/YYJ airspace at 4pm. In busy environs I would make a point of planning ahead.