If anyone thinks that £100 is miserly, then that's their problem.
It seems like the only polite thing to do is absolutely not mention gifts on the invitation. Because despite it being ingrained in British culture that you 'must take a gift to a wedding' we must studiously ignore this fact unless pressed.
Then when the inevitable requests for the gift list roll in, you then have to think of a wide range of gifts to suit all budgets, which when you have had your own house for a while is pretty much impossible, because you genuinely don't want or need anything, especially if you've combined two households along the way.
So say that 'honestly, no gifts are required' and people still bring a load of tat and you end up with a pile of photo frames and glasses that end up in the charity shop because people would rather either spend a tenner on a photo frame and kid themselves that the recipient thinks it is more expensive, or waste much more on an expensive one, because they have this misguided notion that everyone wants to fill their home with glasses and photo frames, seemingly ignorant of the fact that anyone older than about 25 is likely to have more of these items than they could ever need, unless they are a voracious declutterer.
Many other nationalities do this so much better, and just give cash and everyone knows that's the deal.