Hi
I control the sector that you are enquiring about.
You are given FL310 65 before GWC as this will get you into London Upper Sector (LUS) as they have a dfl between London Middle Sector (LMS) & LUS of FL305. Most aircraft can make this restriction and that is why it is in place. Based on a jets rate of climb then this is the level expected to be at this point when you come out of EGCC or EGGP.
You mention changing this to FL290 65 before GWC. This would increase the workload of ATCOs for several reasons. The planes would have to be included in the LMS count & as most of the aircraft can make FL310 then the aircraft wouldn't actually enter LMS airspace therefore the other planes would be penalised & have slots in LMS airspace because of "phantom aircraft."
We understand that on a hot day with a tail wind then you may not be able to make this restriction, whereas on a normal day with a headwind then you may be able to make it. We don't know whether you are going to make a level restriction on any given day. We have a good idea based on temp, wind aircraft type etc.
The key to it all is to tell us ASAP if you can't make the level restriction or don't want to bring the speed back to make the level restriction. Just let us know if you can't make it in good time & we will co-ordinate with the required sector. If you say you are only going to be FL270 then let us know & we will phone LMS. If they have no traffic in the way then you will be told to continue climb to FL31o cross 65 before GWC FL270 or above. If LMS have traffic in the way then you will be told to stop climb at a level below FL310 & work LMS before you work LUS.
It's not a problem not making a level restriction as long as we are told in good time. If you tell us 80 before GWC that you can't make it then there is a lot of frantic last minute phone calls as you are about to enter a sectors airspace when they have no details on you, aren't expecting you & may have conflicting traffic.
Hope this helps.