Experience has revealed that you have to persist with the police if you want anything from them.
Assuming that the MPS are telling the truth then this FOI now finally concludes that they no longer have any CCTV footage relating to 7/7 in their archives, so no more requests to release it need to be made. I noticed that the MPS made no reference to the CCTV footage they released in 2008 that has long since vanished from their website so presumably they didn't archive it.
There is a slight possibility that the NPCC maintains a copy of the CCTV footage from Luton station in their archives although the NPCC are far from the most approachable organisation out there. If they do not have a copy, and neither does Thameslink, then in all likelihood it no longer exists.