First you should know that there is no regulated common market for Forex trading. That means that there is no common ECN price feed, the feed comes from the market and ECN just means that the trade is directly filled the market without an approval by the broker.
It also makes more sense to compare
broker quotes than broker spreads, as quotes can differ significantly in the main market direction. Unfortunately the page linked does not show quotes realtime for all brokers and often does not work. Yes, it's a mess.
No, I didn't test SAXO and LMAX directly. I'm satisfied with Darwinex and their London broker company Tradeslide.
No, I don't know that as I'm not a resident in the UK.
As Darwinex has their licence in London via their trading company Tradeslide, I'm sure they know it much better and they will file only statements to the authorities which they have to file to them. And I'm also sure all the others will do the same.
They all do what they have to do.
If FX trading is tax free in the UK you don't pay taxes regardless with which broker you are trading - highly legitimate brokers in Europe have their licence in London.
As the UK and 96 other countries signed the
CRS, since 2017 it's a child's dream to think the domestic authorities cannot know it when a new account is opened in another country with higher legal standards.