I don't know whether this really qualifies as a loophole but it's a method I use when I don't want to meet the minimum spend for delivery. This is most useful if: A) You are able to buy from both an Amazon fulfilment centre and Morrisons on Prime Now or B) You shop from Amazon a lot.
If your options are like mine you'll be able to choose between Amazon or Morrisons on Prime Now. They'll both offer some different items and I make use of both of them. There are a few major differences. You won't find electronics or gift cards at Morrisons and the gift cards are what we're focusing on.
So if you only want something for £10 from Prime Now's Amazon centre but want to get the free delivery normally for £40 you just add the item to your basket, then add a £20 and a £10 gift card which brings you over the mark. So now let's say you want to do a main grocery shop from Morrisons some days later. It turns out you can actually use the main Amazon.co.uk
website to get groceries from Morrisons. (At least in my experience.)
The Prime Now website is different in how it has its own credit system. So if you buy an Amazon gift card it will not work on Prime Now's site. The trick is to add all of your Morrisons groceries through Amazon.co.uk
and then you can use the gift cards you purchased. You can just rinse and repeat.
I know this won't be useful to everyone but I think it's good when you'd otherwise have to add a bunch of items you don't really need to get your Prime Now delivery. Just get the gift cards. Sometimes it's just useful if you buy from Amazon often enough to make full use of the gift cards anyway.