h and Welcome.
You've got a startlingly good car there! As to the steering, start out by seeing if you can live with it being hevy at parking speeds. The payback is precision and excellent feedback through the wheel when motoring. There are a few tricks to sort out heavy steering..
First off get the tracking optically checked and adjusted to spot on zero, don't accept a setting that is withinn Rover's stated tolerences - they work much better at exactly zero.
Next make sure you have good quality tyres - the best you can afford. It's worth going up a width to 175/75 X 14 too. Starngely this seems to lighten the steering.
Obviously go through all the ball joints etc to check for tired or stiff ones. See if there is any play in the quadrant on the bottom of the steering idler - if there is, then it has probably already lost its oil and will be stiff from this. Solution is a service exchange replacement. The steering box should have almost no play in it at the wheel. If you can swing the steering wheel from side to side without affecting direction then the box is in need of overhaul and adjustment. You should be able to manage that yourself, if not I can direct you to people that can! Obviously it also needs to be topped up with the correct oil.
Don't fit the later 2200 steering box, its much heavier than the one you have currently. It is possible to convert to 3500 power steering though. The steering box is available new and there is a conversion using an SD1 type pump. Give me a pm if you really find you can't live with the manual steering.
Summary - in good condition the P6 steering is the equal of modern rack and pinion set ups - you just have to accept that it won't be possible to swing the steering easily from lock to lock with the car stationary. If you still can't get on with the parking scenario, then convert to power steering, but the responsiveness when motoring is not up to the manual set up.
Chris