In my experience with getting glider parachutes re-packed, the re-packers seem to deal with any make of parachute sent to them.
Don't know anyone in west London, but you could call the gliding clubs at Booker or Dunstable and ask who packs their club parachutes.
Regarding frequency of re-packing, I think 3 years is a bit long. It would ruin your whole day if it didn't perform as advertised!