I am an experienced ammunition reloader in the states; 100,000+ rounds to my credit (I am anal about record keeping). It is possible that submerged rounds can still be live and therefore potentially dangerous. As mentioned, it appears to be a 303 so it is probably military from WW2. Military rounds are weather sealed at both ends (bullet and primer) and tend to retain their viability even with submersion for many years at a time. But its not 100% either way, so of course treat it as live.
Due to corrosion/thinning of the brass, it probably isn't totally safe to fire but realistically the odds are low of a catastrophe as the Lee-Enfields have a Very strong action. I have long been a fan of this rifle/cartridge fwiw.
If you were to throw it in a fire, it would eventually cook off even if it started out as totally waterlogged. But a noncontained round cooking off isn't dangerous. If might bruise you if either end hit you, and of course you wouldn't want to catch one with an eye or the family jewels.
So that's a long way of saying, No it isn't a serious danger. Physically anyway. But with the laws you are under over there, it is Extremely Dangerous and getting rid of one legally (or testing your throwing ability) is definitely a top priority! Congrats on the Gold!