I would always rather purchase a gift but I'm an old-fashioned fool. At the same time, I'd rather not buy things that people don't need and add to the general proliferation of stuff everyone seems to have. (Items which potentially may hit landfill in a few years. ).
I prefer when donations are asked for a specific charity close to the couple's heart. I don't have an issue with helping them towards a honeymoon, however, I'd rather not contribute to air travel or other things I have ethical issues with. (Sorry I am tres boring.)
In terms of the concept of "covering your plate" or however else you want to put this - where this is culturally expected, I would tend to decline the invite. It's not something I am familiar with, but have given money as part of Sikh wedding ceremonies.
Being expected to "cover my plate" effectively means I am buying a ticket to a wedding. However much I wished to support the couple, I'm always going to get bad value - the food is rarely what I would choose in restaurant, scale catering always gives service/cold food/ other issues like that, I have to talk to people who I might not like, plus undoubtedly I would have had to cover my travel costs to get there. Not to mention I am time poor. If this was the genuine expectation, I'd rather not go - whether or not I could afford it.
I see attending a wedding as a big commitment to the couple. It's a show of support and friendship. I would hope they would understand the level of gift I give.
OP - your friend should understand the £20 gift. If not, is she really a friend?