It's not federalism. Federalism builds it's power from the constituent states/provinces etc that grant power to the central government. The US is federalist as it's a collection of states, and power ultimatly comes from the various states. It's a bottom-up distribution of power.

The UK is a devolved unitary state. All power ultimately lies in the central government in Westminster, and some powers are granted to devolved government's for specific regions. It's a top-down distribution of power.

The easy way to see the difference is that the UK government could 100% legally dissolve the Scottish Parliament tomorrow and directly govern Scotland, they'd manage the impossible and become even less popular - but they could do it; because they are the ultimate source of authority. The US federal government could not dissolve Texas, because it does not have the authority to do so.

It's worth noting that a federalist country still recognises the federal government as supreme over the state governments, but that federal government is limited by the powers granted to it. Whereas a unitary country has no limit, it has ultimate authority in the country (to be kept in check by the judiciary ofc) to do what it wants.