thwy are to cover you if you need any emergency medical treatment (incl. sudden toothache, for example)
You can either take the risk of leaving without one in the hope that no such emergency will arise, or try and get one today - used to be available from the post office, but not sure these days....
The E111 gives you access to medical care while you're away - there's a reciprocal agreement between EU countries which means you can use their public health services while you're travelling without having to pay (not sure if everything is free, but it would certainly cover emergencies I think). Very easy to get - just get a form from the post office and they stamp it for you at the counter. It's free. If you've already got travel insurance I don't think you need the E111.
Sometimes it is free, sometimes you pay something and get a refund of all or part. Can avoid you having to find money in an emergency. You do need proper Travel insurance too in case of extended family accommodation due to delayed return of patient or cost of medical ambulance back to UK , for example.
From memory, I think you may need to fill in details like your passport and NHS numbers before taking it back to the PO to get it stamped don't think they're valid unless stamped, so don't leave it til the very last minute. (At least this used to be the case I know they've changed slightly now)
Oooh, lil, you can download the forms from the website where the France information was, then you could fill them in before you go - there is one for the family plus an individual one for each person. I don't think it wanted passport inf but may have asked for an NI number.