Pretty uncomfortable I should imagine!! You would need a decent air bed to insulate you from the pebbles! And i’m pretty sure that there is no overnight parking allowed  in the Streetgate car park.

Actually, small pebble beaches are pretty comfortable to sleep on.

Depends on what you are used to.

I've camped on Weston Mouth beach ( which, IIRC  has larger diameter shingle than Slapton) many times over the last 3 decades. ( 3 times this year).  Always slept well once the odd lumpy pebble is rearranged under the mat.  You'll want good insulation. A foam mat will give this better than a pure airmat. There are plenty of insulated airmats around these days that give best comfort.

And i’m pretty sure that there is no overnight parking allowed  in the Streetgate car park.

As it happens, we were at Strete Gate car park on Sunday evening, and overnight parking is allowed - it costs £3 at the moment (I think fees vary through the season).

However, no sleeping is allowed in vehicles.

And I'd be surprised if there wasn't a local bylaw that technically prevented camping on the beach, though I know some do it occasionally.

I recall girlfriend and I getting in to trouble 45 years ago; being woken at 5.30 in the morning and being told to pack up and move on. Personal details and car reg taken, threatened with all sorts and told in no uncertain terms not to try doing it again.

But that was then, and now is now...