You can’t simply say that cargo can be moved (to East Midlands).
East Midlands has pretty much zero belly hold freight, and these days not much outsize either. East Midlands’ business is almost all courier, mail and parcel - DHL, FedEx, UPS, Royal Mail. East Midlands’ network isn’t that massive either - it has some point-to-point including a few long haul but it is very much fed / feeds through other hubs such as Brussels, Leipzig, Cologne etc, all of which are much larger. Finally, in terms of the type of business it specialises in East Midlands is pretty full - it has the country’s busiest runway overnight and there’s very little apron space available. Sure there’s a lot of room in the day, but that’s not when the sort of cargo that passes through East Midlands needs to be moved - you can’t fly the majority courier and mail in the daytime, that’s not how the business works. To handle a lot more cargo, EMA will need to expand its facilities. The reason why the 747-400Fs that come and go to China most days (Ali / Temu) can use EMA is that they can be scheduled in the daytime.
On the other hand, almost all of Heathrow’s cargo is belly hold. And a fair bit of it is larger / heavier items that the courier firms wouldn’t touch. Being carried in the hold of widebodied passenger jets, it can make a substantial contribution to / support revenue generation. And not all of it is UK O&D either - trans-shipment is in the mix. So most cargo passing through Heathrow doesn’t actually eat into runway capacity or use slots, but it is important in supporting the passenger operation.