I disagree Talkdownman. I think it is worth making a note of some useful frequencies en route. If you drop out of a unit's radio coverage before they have given you the next frequency, who ya gonna call?
To Speedbird91...
Ok so there's always London Information if you can get a word in, but take a look on the map at the airfields whose controlled airspace or ATZ you might want to enter or fly close to. Remember that weather might cause you to deviate from the direct track. It's also worth noting the radar or approach frequency of any airfields you'll be flying within 20-30 miles of in case you need navigation assistance or trafffic information.
Google UK LARS or Lower Airspace Radar Service to see the area of coverage of participating units.
That kind of preparation isn't second guessing the ATC units, its good flight planning. Yes, ATC will often suggest the best frequency to contact next, but I think it's important to be prepared.