Missy
It doesn't happen all of the time, and not normally on the scheduled stuff, but it does happen quite often (maybe once a week) with the biz jets etc that fly out of Luton.
The SID restrictions in this area are very specific because in the Brookmans Park area you have LHR outbounds at 6A, Luton outs at 4-5A, City outs at 3A and closeby Stansted outs climbing to 5A. Once you get your head around it it makes sense, but can be quite unnerving if the pilot reports his cleared alt as 6A, especially when the Virgin A340 is crawling through 4.5A....!