Doesn't the loss of representation in the European Parliament happen at some point in the withdrawal process?
As for kicking the UK out... is there anything in the treaties constituting the EU that allows for such an action? And if there were, and I were advising the EU... kick out the UK, then turn around and tell Scotland that if they vote for independence, they'll get automatic membership. (Scotland's really pissed since one factor cited by the "No" side in the independence referendum was the loss of EU membership, and of all the various regions of the UK, Scotland voted "Remain" most heavily, but got dragged out by England anyway, which had just been telling them that if they did leave the UK, they'd also leave the EU, and now by not leaving the UK, they're leaving the EU against their will.)
And that doesn't even get into the mess Northern Ireland could become if the hard border goes back up and the Good Friday Agreement no longer applies (oh, another region that voted Remain), or the chaos in the financial sector (London's full of bankers and voted... Remain), or how it could impact Gibraltar (which voted something like 96% Remain).
EDIT: Or, in other words, all the reasons the three regions that voted Remain (London, Scotland, NI) did so have been proved valid, and all the benefits that were supposed to accrue to voting Leave have been less than forthcoming or apparent.
Voters forgot that leaving the EU doesn't mean just getting the economic benefits setting your own trade policy gives... it also means giving up all the economic benefits EU membership gives. All too many of them thought they could have both.