How is it possible ?
How is it possible to be VFR on top of OVC ? Surely you entered IMC to get there, or will on the way back down. If you went through a hole then you are not IMC in the UK since it's BKN and therefore you are still "in site of surface" ?
I guess you can be VFR on top during the odd occasion (and extremely unlikely) that your destination has BKN, as does your departure, yet it's OVC in between. How often does that happen ?