The physics for the MIT company is being tested in the SPARC reactor under construction, they hope to run a burning plasma in 2026

If that works they'd like to build a 200+MW reactor by 2035. The UK's STEP design is not as strong as the MIT design so it would have a lower magnetic field, however per size and magnetic field it produces 8x more power. It has some notable disadvantages though which is why it's nice to see both being tried

There are several candidates for the first grid connected fusion reactor that plan to build before STEP. Ironically the doubt about it being the first grid fusion reactor is kind of backwards, it's not that it's so unlikely to work but that it's fairly likely others will work first

The big catch that remains may be whether they can be built to produce commercially competitive power