If the rule were maintained, or not completely waived, the only beneficiaries would be airlines using government money to slot sit. That's not going to be EU, or even US, airlines, but airlines from places such as China which will benefit from a long-term strategic viewpoint. At the current time, I can't see the UK Government, or even the EU authorities, wanting to pave the way for a huge long term benefit for China. Also, the EU won't want it to become easier for the UK to expand links with Pacific Basin economies.