411a..re holding at DVR, not as simple as you state. Inbounds from Europe Don't route via DVR to the LTMA (G1/UG1 is effectively Eastbound only). LTMA outbounds DO route via DVR to Europe, and London City outbounds (not there in 1974) regularly cross abeam DVR climbing slowly thru your proposed hold to Fl120. DVR holding at ANY level is a nightmare for ATC. I know, I am valid on the DVR sector (amongst others).
We still do not have a rational answer to LL DIRs reasonable question. These slow guys outside the accepted 0600 curfew cause a lot of problems to AREA Control as well. I'm also interested in the reasons behind this.
Rgds BEX