If CB's are about on a light IMC or VFR day, consider filing IFR even if you intend to remain in VMC as much as possible. Once you are in the system then CAS busts and navigation worries for deviations become much less of an issue.
Oddly enough, I would argue that one both ways.
It's true that if on an IFR/airways flight plan, flying under ATC control, controlled airspace becomes irrelevant, so you can ask ATC for a "20-left due weather" etc and they will (usually) let you. Even national frontiers, within Europe anyway, become irrelevant, although ATC can get very grumpy if your diversion means they have to make a phone call to the other country's unit...
However, European airways routing levels (MEAs normally FL070+) normally place you initially in IMC, so the strategy is to fly VMC on top where it's nice, sunny and smooth. But you have to get there somehow, and back down again at the other end, and the IMC section cannot be avoided, and if that IMC has embedded CBs then you are in big trouble.
However, if you can do a flight under VFR in Class G (by far the most common UK private flying context) then you can avoid IMC at your leisure, and ensure that you can always see what you are flying into.
I had a very good one last year south of the Alps, where a specific IFR flight would have taken me into solid IMC with embedded CBs, big thunderstorms actually looking at the sferics data for the region, and due to the SID distance would have doubled the flight distance. Whereas a VFR flight at ~ 3000ft was in perfect VMC and smooth.
Filing "low level IFR" (below the airways i.e. not on a Eurocontrol flight plan) in the UK, which an IMC Rated pilot can do, doesn't help at all since the flight plan doesn't go anywhere of relevance. ATC units either file such flight plans as VFR (in which case nobody looks at it unless looking for the wreckage) or they file them correctly (via Eurocontrol) which usually fails because of the problems in filing IFR flight plans OCAS. And it still doesn't help because the pilot doesn't get the implied enroute IFR clearance which airways pilots get.
But yes, accurate means of navigation is a must because one must not bust airspace while flying a diversion. It also raises an issue with notams; if you bust some summer airshow...