Couple of tips/hints.
I doubt that the distance offshore matters, look for a rocky area, drop-off or wreck on your sounder and drop your pot nearby.
Please, please put any berried (females carrying eggs) crabs or lobsters back and respect any local size limits/fishery byelaws. If you are in Norfolk then I think that you are in the Eastern Sea Fisheries Committee area, see:
Buoy your pot, or use a good transect and a grappling hook (better for a line of pots though).
I think that fresh fish works better in some instances, but the advantage of a smelly tin of dog/cat food is that once a lobster is in the trap it cannot eat the bait, therefore the trap keeps on fishing. Fresh fish means it will eat it and they'll stop a coming!
Good luck and (hopefully) good eatin!