Assuming that you know the icing conditions (in the clouds) then is it better(?) easier(?) to punch through the clouds and enjoy the sunshine on top, or to continue in marginal viz in the murk beneath the clouds?
Unfortunately that question is a bit moot in most cases because of UK's airspace structure forcing you low down - unless you have the full IR and then can file a Eurocontrol IFR flight plan in which case you get an essentially unlimited airspace to play in, plus the guaranteed enroute clearance.
Otherwise, I would always climb to VMC on top. This gives you generally unlimited visibility so you aren't going to fly into anything nasty, it gives you trivial avoidance options if there is something nasty ahead (CBs etc), and the main 'problem' is that you have to get back down somehow at the other end and you don't want to descend through 10,000ft of freezing layers for example - definitely not if the 0C level is below the MSA (because the ice may never melt and you will be landing with all that you have collected on the way down). The other problem is that the cloud tops could keep rising and eventually reach your operating ceiling (or, if you have the IMCR, they could rise to the base of CAS which in the UK is likely to be Class A). But a decent look at the weather data should address these things.
The opposite strategy (flying below the cloud) is often practically safer because you cannot get shafted by the cloud tops rising up into CAS into which you don't get a clearance. And if the terrain rises to meet the cloudbase, you can always turn back (can't you?
). Of course this strategy kills many people every year (scud running) but it really shouldn't.
However, in weather like the s. UK has today (freezing down to ground level, especially in the mornings) there isn't much flying one can do safely at all, and that is just the price one must accept for not flying a much more expensive and capable plane. I would want to stay in VMC the whole time, or perhaps climb/descend through some thin cloud.
It was funny yesterday pm. +4C on the ground, falling to +2C at 2000ft, then rising to +6C at 4000ft. That was fine for flying, even in IMC.