I think you might be ill advised to condsider a FIS with squawk as being any better than a usual FIS. Just because they have a squawk from you doesn't mean ATC are doing anything more than checking that you are doing what you said you would be doing. After all a FIS only means you get weather, nav aid info and info on known local traffic. Having said that most controllers seem to give a little more if workload allows.
So back to the original thread, unless you are doing zone transits, how many people should you talk to? In theory you only need to talk to departure and destination airfields, although it would be wise to get a FIS from London en route. Would make the RT much simpler and encourage a decent lookout. On the other hand with so many regional airfields taking procedural IFR traffic, maybe we should be talking to everyone. It is interesting to fly with some ex mil pilots and see how little they use the radio.