Can I directly start a French Masters 2 with a UK 4 year integrated masters?
Yes, your integrated masters from the UK counts as a "Masters Européen"in France.

Source: I directly began a PhD immediately after finishing my MPhys in the UK, at Paris-Saclay, last year. If I was able to do that, I have doubts (although it's possible) that you would have trouble doing a M2 here.

As others have said, and as the other answer said, you should contact the university / admissions team as a first contact, to verify this. During the admission process you'll have to send your transcripts, and so in the end, they'll decide this for themselves. If it's anything like my admissions process, you will also have to send your MPhys dissertation, because it proves that you have done M2-equivalent work and a project with a significant time commitment and research result.

However, I'd be very surprised if you had any problem.