Nobody says you can't, carrying whatever amount across the border is perfectly legal. What's illegal is trying to carry >10k without declaring it to the customs.

If you're a law abiding citizen, why wouldn't you declare it? If you don't, well congrats, you're no longer a law abiding citizen.

They have but it needs to be declared so as to avoid tax evasion and money laundering

Whether or not our citizens should have the right to do something seems rarely related to whether the UK government sticks its noses in our business to make sure we aren't.