In the UK, with an IMCR you can fly VFR above a solid layer, so if you are in VMC you have the choice of VFR or IFR.
It's normally better to reply "IFR" because you are more likely to get a radar service, and a handover to the next radar unit.
Then, if you do enter IMC, you remain legal. Makes life simple.
The only strong reason for flying "VFR" (that I can think of) is if above 2000kg, to avoid IFR charges. I know of people who fly big twin turboprops, up to FL195 in France/Spain like that. It's a bit of a mick-take but if it's Class D...
The other thing is that if you want to
arrive IFR i.e. on an IAP. One is going to get a "better reception" if one was IFR on the way there... if for example you go to Cranfield, VFR, and tell them you'd like an ILS they will tell you rather bluntly they are busy with instrument training and if you really really still want the ILS you can have it but they will charge you for it (meaning: "go away, come back VFR and report when you have found Woburn Abbey"). Whereas if you flew there IFR they can't do that.
No doubt 10 people will now jump in, saying how much they wish the UK had the same airspace structure as France/USA/etc (Class E nearly everywhere) all flight in IMC should need an IFR clearance, and all real pilots should have a JAA IR
