Evening Bookworm.
Just a quick comment in addition to the above. I work the TC South side which you would have entered about 10 miles before CPT. I note that others have noted that the EGLL SIDs climb to 6000' and depending on the pressure FL70 may not be separated.
A further complication is that in this area Southampton departures have to climb to FL70 (to stay inside CAS after departure), and Luton departures have a 'Min Stack' standing agreement (usually FL70 or 80 (and yes exceptionally, like this week, FL90 - but that only happens about once a year)) so FL90 is the lowest level that can be accepted 99% of the time without a disproportionate impact upon normal operations.
I can see your point about the low level STAR but it can still have a use. If traffic levels are low (an increasingly rare event) then you are likely to be accommodated at FL80 or possibly FL70. Also if a special flight with a high category is filing that route then you can, and will, place restrictions on 'normal' commercial traffic to allow for the single plane.
I hope this helps but feel free to PM me for further discussion if you like.