It is not a matter of one or the other, just a little flexibility and awareness that not all crews actually want or need to be as low as they currently are being routed. The 3 "gates" MARGO, TARTN and LANAK are too low and especially so for days with a northerly wind. Most of us know there are reasons for the "gates" but some of the reasons for those levels are not necessisarily relevant to current ATC or aircraft requirements. If they could be tweaked, even at off peak, there could be some worth while savings.
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ATC are aware that all aircraft would like to be treated as the only aircraft in the sky and fly without any speed, route, or level restrictions ..... but meanwhile, back in the real world .... the system is designed to give the majority what they need and, as someone mentioned, that's been an aim of zero delay for a good few years now. Airlines stress the importance of getting things like the first rotation away on time, because otherwise their schedule for the rest of the day is screwed up. Airlines stress that they will take a hit on economy, by flying around, over, or under ATC sectors where there is delay. The passenger wants to be on time !! Therefore the system has been designed with that in mind. It means there will be winners and losers but it is not possible to provide a bespoke service so that everyone gets what is important to them. Th overall need is the one which takes priority.
The MARGO level restriction exists so that the aircraft is descended out of the Dean Cross North sector (when open), and has a secondary effect of getting it out the way of any overflights routeing NEW-TLA, the latter being an H24 problem. When Dean Cross sector is combined and traffic demand is lower, the restriction is often lifted (usually from early-mid evening until the following morning or at quiet times during the weekend).
The LANAK restriction ensures that inbound aircraft are descended below traffic climbing out of Edinburgh on the SID towards CUMBO, which has to make FL100+ by CUMBO. The alternative to the restriction would be to keep Edinburgh outbounds at 6000' until about 40+ miles after departure, amend the Glasgow SIDs to 5000' to avoid the new confliction with the Edinburgh traffic stuck down in Glasgow airspace, and keep inbounds at the FL equivalent of 7000' until well clear of the Edinburgh SID track .. probably giving you 20 track miles to lose that height to touchdown. Do you reckon there would be a bit of bitching from pilots and airlines alike about that scenario ??
The TARTN restriction is the only one with a little scope, but again you are going to have to be descended at or below 6000' to remain clear of Galloway sector airspace which is about 10 miles North of TARTN. Edinburgh controllers posting on here may have a view on that.
I can't agree with you that the restrictions are not necessary to current ATC requirements. They very much are in place to precisely meet ATC needs and increase capacity. There is nothing to stop them being reviewed for a change if that's what the industry wants, but there would be an impact and the potential for capacity to be affected, in spite of what you say. It would all depend on what the change was.
You'll probably have noticed that the inbound STAR restriction to Edinburgh and Glasgow of FL180 30 miles before the inbound fix is only applied occasionally. It's there to facilitate a sector split and is only used when that is in force. So we can, and do, provide flexibility when we can and don't stick to the rigid published level.
If you depart any Scottish airfield on a SID, you'll also find that you'll get short cuts whenever we can as well, either with a direct route or a short cut vector.
Add in the extensive direct routeings given to high level traffic at all times of the day, and you'll find that there is a high level of awareness amongst the guys and girls here about providing efficient flight profiles when they are able to.