My .02
At Heathrow it means exactly what it says...comply with the noise abatement listed on the SID. Cross the noise terminal at or above the listed altitude and thereafter maintain a 4% gradient at a minimum. A 4% gradient at 250KIAS is slightly more than 1000fpm so it should be no problem using your operator's normal climb procedures.
Having said that I've seen pilots elect to use normal NADP's as a technique to ensure compliance.
I believe either is acceptable.