Nothing to do with Brexit, since 2015, long story short not that much: if you factor weather & schedule restrictions it gets tricky to fly UK to Paris…
The best option I found hassle free is fly to Lille (60€) then 10min uber and 1h TGV (30€ O/R tickets)
I have flown twice to LeBourget, both were less than 1h parking and it was +500€ for 1.4T
Beauvais is one option: if RyanAir lands you are guaranteed a quick bus to Porte Maillot (there is train as well) but make sure you don’t need any Avgas when RyanAir lands, they will be busy serving their masters first !
The other quick alternative is Rouen for customs (get PNR right) and then Toussus, maybe worth asking ATC if it’s really necessary to stop at Rouen? this may have changed with Brexit (on flight without pax, they give you departure clearance for Toususs before you start approach to Rouen), if you can take grass, StCyr is a very nice & cozy option, then very close to train station and straight to city center
Going VFR to Toussus is tough, don’t do it, unless you know the place by heart and your first time was J3 Cub
Going IFR to Toussus is easier, you will be puzzled by routings from Rouen/LeTouquet to Toussus, I suggest just file something that validate and ignore: ATC will take care of it, alternatively once you cross CDG/Orly axis, ask direct Toussus but let ATC know explicitly that you are happy with uncontrolled IFR (they don’t offer “du IFR non-controllé” out of the blue, so they may route you all the way south with Orly arrivals via ODILO+STAR+INA RNP+Radar+ILS to keep you in airspace), if you get direct Toussus, the ILS25 is very scenic: you fly direct overheadToussus, then outbound on back-course LOC (now TSU VOR070), then back on ILS, if it’s VMC, you will fly near Eiffel Tower twice