My suggestion would be
On a white car, if you complete your normal wash (which sounds fine btw) and then:-
Tar Remover sprayed over lower half of car and boot lid (in cool temps and shade) then spray more onto a cloth and go round the vehicle and wipe the panels, you'll need to focus on the bigger tar spots a bit more.
De ironiser over the whole car, leave to dwell for quite a few minutes (you'll have lots of purple runs), then rinse thoroughly
Clay the car thoroughly (there shouldn't be TOO many contaminants left at this point)
Rinse the whole car down and dry
Apply a thorough layer of SRP to whole car
Apply a SEALANT/wax to the bodywork (collinite 476 is my choice for durability,shine per £. apply to a small area and buff straight away)
This is obviously all my own opinion but i've done this many times on white cars and everyone has always been thrilled with the results, if you dont want to do it all, i'd drop the deironiser first then the separate wax, SRP does leave a protective layer but adding another proper wax/sealant will mean all your hard work will last much longer, and washing it will be easy as anything.
Hope this helps, let us know how you get on!
Alex
ETA: Use the tar remover and the deironiser on the wheels and it should take care of any black spots