I agree with the OP in as much as this was 'plain stupid' - but criminally negligent I'd think not. Class G is see and avoid - even if the intrepid three were not aware of the NOTAM info it is still their duty to lookout, just as much as it is that of the aerobatics pilot.
Should aerobatics everywhere be NOTAM'ed? I've seen plenty of aircraft practising aerobatics all over the place with no NOTAMs to speak of; is this more dangerous? Should aerobatics with no NOTAM be classed as 'criminally' negligent? What of the pilots who fly through an area where another pilot is doing aerobatics. Are they 'criminally negligent' in the fact that they aren't under a traffic service? What I'm trying to say is, this term 'criminally negligent' seems to be a catch-all these days for anything that could annoy someone else. "Oh my God, I could have been hurt there, now where's the number for my lawyer...? Ah yes, speed-dial 7" (This is not a dig at you, DX Wombat, more at what I think to be the litigous nature of society today)
As for sending an apology to the pilot and BAeA, again why? Class G. I could NOTAM a piece of airspace tomorrow because I want to practise some aerobatics, but I have no legal ability to force people to avoid it. You may have their registrations, but short of creating your own personal 'black book', there's not a great deal more that you can do.
However, I feel that I must add that I do feel that what the intrepid three did was very stupid, and only served to increase the probability of being instantly welded to another aircraft. There is no substitute for proper flight planning and sound airmanship - both of which has not been shown here.