They wouldn’t get any cash from me. It’s not my job to pay for their honeymoon. What are they going to do if it’s not enough?
They would get a John Lewis voucher from me
But why ?
What if they don't shop in John Lewis / don't want anything from John Lewis ? 
I haven’t been to a wedding for years and I have never given money as a wedding present.
It sounds like these days, guests basically pay to attend the wedding.
Don’t people give gifts / have lists anymore?
Some do, some don't, but life today is different from even 30 years ago, but definitely from 50 / 60 years ago.
Most weddings I went to in my youth, people were setting up home for the first time, and people traditionally gave gifts for their home. More often than not now, people are already living together when they get married, so already have all the saucepans, towels, an iron, a toaster, crockery, cutlery, etc etc, that were typically on wedding lists in the 80s and 90s, but they know that most people wouldn't want to attend a lovely but expensive celebration without giving a suitable gift. Rather than ending up with 30 bottles of champagne or silver photo frames, people will usually suggest to people what their next 'spend' will be and people can choose to contribute or not. It might be a honey moon or it might be a dining table. It doesn't really matter, except it ought to be what the recipients want and not what the person giving thinks they ought to want.