javelin.
You are getting confused by 2 different procedures. The 70 before WAL bit is an ATC matter to ensure that you:
a) don't infringe othere peoples' airspace and
b) are on a profile which will enable you to comply with the minimum stack level AND 250 kts at the Speed Limit Point.
This was also the reason that many of the other level restrictions were introduced eg FL250 at LOGAN and FL150 at SABER. There were many operators who were paying no attention to the SLP and were doing 320 kts to get the height off and then infringing other holds due to the high speed.
The 50 before MID bit is also an ATC requirement to get you into the London Upper Sector(LUS) and above the Middle sector(LMS). If deleting the speed restriction is the reason you can't make it then tell Manch that the speed will be low to get the height on. They are cancelling it for your benefit usually.
Not making those sort of levels causes additional workload for the ATC system and also has an issue on Flow rates. If you are Requesting FL330 then you are flowed in the LUS rate and not LMS,but not achieving FL305 or above potentially puts you into LMS. Maybe if you are on a long route in an A321 etc then it would be better to file an initial FL below 300 and be included in the LMS rate. At least then if you are doing well LMS can offer you to LUS and be accepted depending on the traffic situation. The other way round LMS has no option but to have the traffic go through the sector in the climb.
Hope this makes sense and helps you in the long run.