That's exactly the point of the P6. Soft long travel suspension, well controlled and with good geometry trumps even the best circuit racer every time on real roads. Sort the roll with a roll bar or two and you have the ideal set up for the UK's fast rural roads.
HAve a look at the layout of the throttle linkage in the reference section of Ian (Rover-Classics) web site and you'll find there are a number of rubber bushes in the throttle linkage, most of which are prone to go missing. The most important ones are the one attached to the off side bulkhead where the linkage changes direction from side to side to fore and aft, and the one where the pedal passes through the bulkhead. Getting these right is the most effective method of tuning the engine as it allows you to achieve full throttle....
Once done, you might need to adjust the kick down cable to the transmission. f you've tightened up the linkage it will probably be too tight and won't let you change up early enough. Also it will exacerbate the clonk going into drive. As a rule of thumb, it should be set to the slackest that will still just give you kick down. Then the other measure to reduce take up clonk is to get the engine to idle as slowly as it is happy to. The book value is 650rpm, but if you can get it to idle happily at 500rpm that's even better.
The BW is an unusual box to drive. I find I tend to drive it like a manual - that is, still in D but throttling off momentarily when I want it to change up.
Chris