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"They generally don’t pay NI, so taking the Employee and Employer parts that’s roughly 25% rate not paid."

Not really true. The full calculation is this.

Top rate - salary you take (which has all PAYE, NI etc paid out of it) - applicable expenses (ie, the contractor paid them) = gross profit to the contractor.

Next, how to get that profit out of the company into your pocket and how much will that cost? The answer is as a company dividend.

Gross profit - corporation tax (20%) = net profit/ dividend to the contractor as personal income.

Then, you lose 10% of the dividend in the contractors personal accounts as another tax.

So, money extracted via dividend has been taxed at 28% to get into your pocket (20% corp tax, then 10% of the 80% left to you)

Then, in your personal tax affairs, you need to take into account things like tax credits and other fun things to determine what the optimum salary actually is. It used to be just the free tax band and no more, but now its more nuanced and is probably about £30kish.

All told, the only tax that really goes missing is the employer side NI in the second part of the equation. A standalone ltd company contractor will pay about 30 - 35% effective tax to get the money into their pocket (I can't be bothered to go through my own accounts to tell you the precise figure). Really, not much different to an employee, considering you get no holiday pay, sick pay or any other benefits from an employing organisation. You also need to keep at least 6 months money in the company, all the time, or you'll go hungry one day, guaranteed.