im sure the club ropes will be ok for the clip drop technique, they'll be more than safe enough. but i wouldn’t do it excessively on the same rope for a long time because you'll just shorten its life unnecessarily. the club does have some ropes it uses predominantly for indoor use, perhaps use one of these.
basic considerations for ropes/checks you should do periodically:
> retire after 5 years from purchase
> for a club rope used frequently, it should be retired after 2 years
> unsafe if there are notches in the sheath, or excessive furring on large sections of the rope
> consider retiring it if the core has come away from the sheath along large lengths, or the core has shrank.
> check for lumps etc in the core along the whole length, these could be weaknesses
you are correct about 'impact force ratings'. lower is better in term of reducing the force on climber but tends to mean the rpe has a sorter life span - read this from the BMC
http://www.thebmc.co.uk/Feature.aspx?id=1440you can also download theBMC's Care and Maintenance booklet
http://www.thebmc.co.uk/Download.aspx?id=77