More reasons then from the route you gave first:
KOK-KONAN-DVR is opposite direction to all the London TMA traffic departing via DVR so giving you a heading nudging you slightly south of track gives us the best amount of space to get the TMA traffic quickly through your level. The LCY traffic will be passing FL100 somewhere between DET and DVR as is the Stanstead & Luton departures. Next you cross N613 between SOVAT & SANDY, all LCY, Biggin Hill, Southend and Rochester traffic have to be FL80 by SANDY, if you are on your track in conflict with one of these arrivals, we cannot achieve FL80 by SANDY unless you are vectored slightly southwards. The next track you conflict with is again traffic into LCY etc inbound along G27 (HASTY-BEXIL-ROKOS-SANDY that have to be FL100 by HASTY. Being your level, you will be vectored off track to avoid this route. Next, as you approach SFD (or 10nm before it) you will have a steady stream of LGW arrivals crossing from right to left attempting FL70/80 around TIMBA, again, if you are on track, that is not achievable. Once you cross that track, then depending on the LGW runway direction, you could be in direct conflict with departures if they are on easterlies. If on westerlies, then you will be in conflict head on with LGW arrivals via GWC-HOLLY-WILLO which also need to descend through your level.
For all those conflictions, you need to be on a heading to ensure separation or to tactically position you so other aircraft can achieve agreed levels. As to why it, in your opinion, only happens in the London TMA, then I guess that we are just that busy that you have to be.