Cloud tops
I do find myself envying the rich guys in Barons with weather radar, but this kind of extended CB activity is less usual that just yer regular low-level clag.
As a holder of a year-old IMC rating, my more common concern is the cloud tops, which are very hard to establish. I know that 1000' cloud will inevitably mean some IMC work leaving under the 2500' Class A that sits over my N London airfield, but having cleared that, if I was able to climb into clear air at, say 4,000' heading west I'd be out of it within 10 mins. But if it was XXX all the way to Wales or Cornwall, that would mean a great deal of hard work and, with no TCAS and without even a radar service in parts, I may not feel entirely relaxed flying blind all that way. But of course the 215 is far too general to give any real planning accuracy as to cloud tops over that distance.