VFR on top is permitted in UK airspace. However, it may not be permitted on your licence. A UK issued "vanilla" JAR PPL or CPL holder cannot fly VFR on top, but an IMC or IR holder can.
Can you please point out where "VFR on the top" is requireing IMCR/IR? As far as I understand it (and the attached picture shows it) the "in sight of surface" is only valid if under 3000 feet and clear of clouds. The other option ("or" from the attached picture) is 5km vis, 1000 feet vertical and 1500 m horizontal from clouds (no surface in sight mentioned here) for VFR flights (i.e. no IMCR/IR needed) - all under FL100. I'm reasonably confident this is also the case in other JAA/EASA countries (where IMCR doesn't exist).