A Different Kind of Planning

From the way this post will ramble, you probably will be able to tell that I can't decide on anything right now :)Usually I am the type that has all my hotels pre-booked, and have a daily itinerary planned before I arrive. This trip, I decided I wanted more freedom. However, this makes it harder for me to plan. I arrive in London on Saturday, March 31. I would like to spend Sat, Sun and Mon in London. I was thinking about taking half of one of those days to run down and see Hever Castle. (This isn't my first trip to London, so I have seen most of the sights already.) Then I know for sure I want to end up at the Pembrokeshire Coast Path, and spend a couple days wandering around there. I also would like to stop on the way to see some cathedrals like Worcester. I was thinking of renting a car. I'm not afraid of driving, I'm just wondering about the cost, parking and time spent on the road. I also considered taking the train, but this would give less of the freedom that I wanted. Also, I don't think there will be enough time for this, but I would also like to go to north Wales, and see castles like Criccieth if I could work it in somehow. I leave from LHR the next Saturday. Or I could go to Wales first, and then spend the last couple of days in London. Help! Any pointers, suggestions or even anecdotes that would help me focus this a little more would be appreciated.