Hmm. Don't really want to comment on the specific case; I'd need to hear all sides of the story before I could sensibly make any judgement.
Suffice to say that it's not a good idea to go anywhere near (never mind actually in) controlled airspace without being in contact with the owner of that airspace. Airports like Luton are very busy with large, relatively unmanouevrable aircraft, and most of those aircraft now have TCAS. Many light aircraft have non-height-encoding transponders (if they have one at all), which cause chaos when near a busy traffic pattern. To explain: if you're on a course which takes you within close proximity of the projected course of a TCAS-equipped aircraft, that aircraft will 'see' you and will have no idea what height you're at - so he will want to change course. If you are identified to the controlling authority, they can reassure the TCAS aircraft that all is well.
If you have a height-encoding transponder and stray near a pattern, you could actually cause worse chaos, as TCAS aircraft will be being instructed to take avoiding action by their kit. Getting those aircraft back in the pattern after an RA (Resolution Alert - which requires a height deviation to resolve) is a major headache, not to mention the real risk of collision!
So, 'free' airspace in UK often isn't really, and that lesson should be being rammed home by FIs loud and clear! Last point I can draw from this: good lookout has saved many lives, poor lookout has lost many. Never take your eyes inside for more than a split second when you are flying VFR; it may save your life.