You have to convince Labour that this is a good idea. Because that's the limiting factor here. Conservatives aren't going to support this. Labour needs to get fully behind rejoin for this to happen.
They won't.
You then could vote Lib Dems into power to rejoin.
Or it might just work if you search for 'Rejoin' now anyway. It looks like to connect Feddit.uk to that Kbin community, someone needs to search for it first with the full url (which I just did with my Feddit account), but once that's done it looks like it appears in search.