I've usually got a little list going of top-up items that I pinch from when wanting to go into the free-delivery window, but if you've not got something like that yourself then there are online tools available where you can enter the price-required to reach £20, and they'll list items for that price that you can then choose from to top-up your basket.
Here's an example that also allows you to categorise your top-up searches -
http://www.fillerchecker.co.uk/I use extensive use of the free-delivery option, and find that offer, along with my local Amazon postal lockers, copes with about 90% of my online-purchase requirements.
It's no shock to me that many high-streets are suffering...
Cheers,
Itsallaguess