There are a couple of threads on this subject, I think most agree that as long as the items are securely stored in your hold luggage (wrapped in sleeping bag or similar), then you will have no problems leaving the UK.
I have had a number of problems at foreign airports where it is sometimes the norm is to have people open up their baggage before they enter the terminal building. These airports were Paris CDG, Amsterdam and Budapest.
Never try to travel via Eurostar with anything sharp in your luggage, you will lose the item and may be arrested.