When taking a couple months of unpaid sabbatical in the UK, the
employer will usually drop all benefits until the employee is back.
Quite certain this policy would cut the national health insurance
What are the options to maintain cover by health insurance during the
sabbatical ? Not asking about the private health insurance, but the
one that allows access to NHS services.
It seems you are misunderstanding how access to the NHS works. Yes, your employer will pay money to the NHS on your behalf (payments to National Insurance). However, these payments, despite the name, are not like traditional insurance premiums - instead, they are more like a tax.
In particular, as Patricia Shanahan explained in a comment:
Access to NHS care is based on residency in the UK, not on paying contributions.
So it does not matter that your employer will no longer pay National Insurance for you, you are still eligible for NHS care.
Would a travel insurance policy cover health costs in the UK too ?
No, travel insurance typically only covers treatment during travel (that's why it's usually less expensive than regular health insurance). Additionally, it often excludes health problems that existed before the travel, so if you fall ill, you cannot travel just to be covered. Read the policy for details.
At any rate, you do not need cover from your travel insurance when you are in the UK, as you are covered by the NHS, as explained above.