The one thing I've learnt about the CAA (including ATC's in zones) is that it's always better if you tell them about your errors before they come to you (if you have to tell them at all).
Between you and Cranfield, you did the right thing in realising your situation and taking the most appropriate action to rectify it. In this case, calling up Luton.
If you'd have just bimbled around a bit, interfered with Luton traffic & then found your way to Cranfield, both you & your instructor would on the receiving end of the CAA's wrath.
As it happens, to them, your probably just a statistic. Which in this case, is good.