We do actually have horizontal sectorisation, Cowly (133.07) narrow as it is has to share this airspace with Cowly west (original name or what) and likewise on the northbound side. One of the problems on Cowly is that at times you just get squeezed for space so even with the frequency split you have the same a/c in the same area.
Level capping unfortunately wont help as if you cap southbound traffic it ends up in the middle of the already congested TMA and how will you get LL and KK traffic down?? Similarly going north you need to get Birmingham traffic down through outbounds once you've done that there is not that much left in the way, just getting everyone the right way round. If we dont climb you northbound you run into manchesters airspace.
This obviously is a difficult problem to solve and people are trying to find a solution more airspace etc.
(Yesterday during one of those very busy sessions there was a Pan call it got in on the RT ok and wasn't a problem, had there been much more traffic around at the time RT congestion may not have been the biggest problem but finding a gap to get them through into the TMA)