I think one needs to distinguish between
1 - possible noncompliance with paper rules
2 - actual loss of safety
We don't know about 1; only the pilot would have known that. Cloudbase can vary from one spot to another.
Also what is the difference between an IFR and a VFR transit? There are some ATC rules about separation perhaps, but ultimately the difference must be that in VMC the pilot is supposed to be visual with other traffic. What if he isn't? Most "other traffic" is never spotted anyway, and if you are not visual with all of it then you are not visual as far as safety is concerned, are you?
If this was a radar unit then the distinction is particularly moot as far as safety goes.
It wasn't in sight having just disappeared into/behind a cloud
That is usually legal in the UK for deps in Class G. The moment you are outside the ATZ (typ. 2nm after takeoff) you are entitled to change your mind VFR-IFR and vanish in a cloud. This is commonly done. Most Class G fields are non-radar and are thus unable to offer any meaningful kind of separation/service. If ATC, they apply procedural separation from traffic they know about. If non-ATC, you are on your own anyway.
Obviously people should not declare "VFR" when they are in solid cloud but what matters is whether safety is actually compromised.