I don’t know of any Morrison’s sites that let you park overnight tbh. AFAIK only Willow Green has a manned gatehouse so you could probably chance it. I personally wouldn’t.
I remember Burton Latimer security being a particularly arsey bunch tbh. I once turned up there in a wrecker towing one of their dead artics and they wouldn’t let me on site without a reference number!
I dropped the whole lot outside the gatehouse and buggered off! My how we laaarfed.