To be fair that is not much time at all.
True. So tell me, how often have you had a current TAF forecast good VFR for an arrival and turned up to require an IFR arrival and/or approach because it was not possible to descend somehow/somewhere to establish VMC below? I think it would be valuable for us to hear about these truly unexpected IMC instances. Note: I am not asking or judging what you did as a pilot in those circumstances, or trying to draw you into anything. I am sure you executed the best safe and legal course of action. I am only interested in the facts about TAFs and actual weather, in scenarios that relate directly to Nick's question - a UK departure to a French destination in a G-reg airplane and holding an IMCr. My modest several hundred hours and 50plus arrivals in this exact scenario has zero such examples, which is why I am eager to learn.
PS. I just noticed IO540's experience is similar to mine: (my underline)
What one does is make sure the tafs and metars for the destination are SCT or better. IF this fails (and I don't recall it ever failing for me, largely because most of my long VFR trips were to warn southern countries) the official legal procedure is to declare an emergency and ask for the IAP.