The max FL270 restriction at liffy is in the MACC SHA LOA and is there H24 for all traffic landing in the MACC TMA.
The restriction to be level fl270 20 prior to liffy is to ensure that you are level, and dont infringe on londons airspace on the descent. It also gives us a little bit of room with traffic climbing westbound. and EI departures that we have to fit into the mix.
These restrictions are a part of life, and unfortunately not popular at times with pilots and controllers.
The one thing i think that all of the restrictions have in common is that there seems to me, to be very little information on the ATC LOA's between the various ATC centres passed onto the airlines, often they seem to come as a surprise to the pilots when we give the restrictions.
Some restrictions, like the london TMA arrivals at or below FL330 at STU, can be lifted by local agreement between controllers, depending on traffic.
The upshot i think is that they are here to stay, and only increase in the future.