18000ft has no meaning in the UK or Europe.
The USA has picked it because above 17999ft is a uniform layer of Class A (to 59999ft I believe, above which it is Class E).
VFR cannot go into Class A, and they have mountains reaching about 16000ft, so it was logical for them to do it this way.
In the UK, 18k would mean nothing. There is Class C at FL195 I think but that's about it.
France is closer to the US model, with Class D at FL115 and Class A at FL195. But the UK is just a mess.
Equally, the 3000ft TA is meaningless. It should be a lot higher, because "flight levels" that low down cannot be flown while respecting MSA, in many places.