I agree with all of the above. The fact that the fuse fitting is there at all very probably indicates that it is live. At one time, the 'LL' stamp actually indicated 'Live Load', although this is not to say that this is the case here. British-made ammunition and fuses normally use a two-letter, and sometimes an additional single number, to indicte the factory which produced it, for example, 'RG 2', which was Radway Green, section Two. I don't know of any factory markings in the 'L' series. Additionally, an innert training round, for loading and un-loading drills, would have a totally blank, 'solid' part for a fuse, just to make the round/casing complete.
Take the whole thing to either your local Police Station, or contact the EOD, found under 'Army' in Yellow Pages. They will 'make safe', and if you ask, you should have the innert round returned to you. It's possible that there may be complications via the Police, as it is technically illegal to posses an explosive, or potentially explosive or converible to explosive device in the UK. However, given the circumstances, after the inital 'formalities', you should be ok!
BTW, it's possible that this was from a 20mm mounted on a late Coastal Command Lancaster or a Shackleton, both of which used the Oerlikon pattern guns in the top turret, and in the nose on the Shackleton MR3.