I don't know the Basle/Mulhouse Airport but at Zurich Airport there is NO passport control for EU passengers annymore. "Schengen" as MacPhisto wrote is signed and since end of march in duty. BUT England is not a "Schengen" country. BUT...
Here are some informations in english. (FAQ's)
[
www.euroairport.com]
Can I cross the border between France and Switzerland inside the airport without a ticket?
Yes, you can cross the border between France and Switzerland inside the airport terminal, even if you have no ticket. You just need a valid passport or identity card. The cross-over point is situated at Departure level (level 3) and is open from 4.00 am until 30 minutes after the last arrival (approx. 00.30am).
However, there is no possibility to cross the border between the carparks.