Bear in mind that permie roles usually include pension, private healthcare, sick pay & holiday allowance. So the total comp from the permie role goes beyond the take home.

45 weeks * 5 days * £300 = £67,500

Outside London as a senior dev you might expect £45k full-time + pension + benefits.

So definitely not at least 2x.

45 weeks is kinda the rule of thumb of contractors, as you don't get paid hols or sick days.

That's true. But 45 weeks * 5 days * £400 = £90,000

500 * 200 days = 100k of income.

100k is definitely not twice what you get as a permie in London.

No, I said it's twice what you'd get outside of London.

Well, the 500 is consulting rate for inside London. The comparison doesn't stand then.

No, it's £300-400 per day outside London. Which at the top end is literally twice a permanent salary of £45k