Hi curiosity,
It does'nt sound like the healthy tree it used to be, normally I would expect the leaves to be still green at this time of year or maybe just begining to turn brown, but not grey.
As these are normally large trees, I would'nt think they would be too happy in a pot for an indefinate period but fine for the short term.
They can dry out very quickly in pots, could this have happened ? (this could make it go into early autumn mode)
You could try a bit of T.L.C to help it recover with a mulch and some mirical-grow but I suggest a back up plan just in case it fails. Over the next few weeks the conkers will be maturing and falling from the trees, so you could collect a few and pot them up and start again, put the pots where squirrels can't find them and they should germinate very easily and you can choose the best one. It wont be the same try of corse but is'nt it the thought that counts ?
Best Wishes. Tbird+