Is this legal?

[guinea36]

I was wondering if there are any employment lawyers or other experts out there who could give me their opinion?
I'm on mat leave at the moment with DC and shortly will start discussing my return to work with my employer.
My job involves long hours and lots of unpaid overtime but I love it and don't want to give up.
Ideally initially I would like to work four days a week and slightly restructure my hours so I come in earlier but leave earlier.
Anyone who in the past gone on mat leave has managed to get these hours on return so it's possible that I would be able to negotiate the same.
However I recently discovered that in order to get these hours most women have had to go freelance and give up their status as an employee, along with accompanying rights etc. In all other respects they are treated like employees with the same expectations. Ie they couldn't not come in one day because they working at a competitor. They may also receive some paid holiday and pension contributions- although I'm not sure.
However I think because they end up paying less NICs they are often financially better off on paper than if they had remained an employee so accept these terms.
I have a feeling that I potentially may be offered something similar on my return. However this arrangement is not going to be possible for me to take and I would effectively be forced out. I'm the main earner in our household and we want to remortgage our home soon. I won't be able to get a mortgage without a few years of proving I have worked for myself. Plus its also a matter of principle for me. If my employer can offer these hours with me as self employed, why can't they offer it to me as an employee?
Can anyone tell me whether this arrangement is legal? Can employers ever offer anything like this and for it to be legal? And how can I counter it if this is something I'm offered?

[Hoppinggreen]

I am not an employment lawyer but I am an actually Freelancer and in order not be be counted as an employee I have to fulfil a very specific set of criteria.
Given that I would be very surprised if you could do the same job with the same employer and suddenly become self employed - it's not a choice the employer makes. HMRC are cracking down ( look up IR35) so if your jib doesn't fit the definition of self employed as per their website than you aren't and the employer can get into a lot of trouble