I'm really sorry to hear you've had these bad experiences.
I also live in the UK, but when I shopped for diapers here, there were no diapers on the shelves in either Tesco, Boots or Lloyds, only pull-ups. I bought adult pull-ups and plastic pants from these stores and I have to say, I wasn't aware of any reactions from other customers or check-out staff. Maybe it's because I just didn't notice, and didn't care anyway!?
Obviously I'm not referring to the verbals and giggles you encountered (horrid ) but if you're embarrassed (as you describe) when you shop for these things, it's easy to be supersensitive and imagine that all the looks you get are related to what you're buying.
I'm not saying you're imagining people's expressions or behaviour, but maybe in some cases you could be misinterpreting them because of your own feelings?
I hope things gradually get easier for you on your shopping trips.
D lly