I agree, it doesn't really matter what they were carrying on board the aircraft, as long as they don't make any attempt to penetrate UK airspace without clearance. If they did that, then it's a different story, but so far I've seen no evidence of their activity suggesting hostile intent, or conspiring to commit a hostile act at a later date (news articles aren't really evidence).
Weapons get tested all the time, and the RAF are not squeaky clean in that they don't have any weapons (or something that can be construed as a weapon) fitted to the pylons in international airspace, when they aren't going to war or returning from war. Look at the Typhoon pictures from this year when they were en route to the USA. Each aircraft had an inert weapon system on at least one of its pylons, so what's stopping another country from doing the same? It's perfectly legal to do as long as you don't attempt to enter another's airspace without permission, and your actions aren't threatening.