Leave it idle and rent after March next year and set the market rate 2k pm or whatever....that's my understanding
Yes, that is my understanding too. If you leave idle and rent in March, then the new rules apply and you can rent at market rate.
But keep in mind:
1. The rent only increases annually at 2% or inflation if lower for the next 6 years if the tenant stays in the property
2. It may be difficult to reset to market rate at year 6 - the tenant could fight it, there could be a rent freeze in place
3. If the tenant remains, you cannot get vacant possession to sell before year 6. Your sale price if you have to sell early will be much less because of this.
So you need to decide whether you are happy to remain a landlord for the next 6 years at market rate plus 2%/inflation. I'd expect the inspection regime to ramp up as well, so that would add to your costs.
Plus if your new tenant is difficult, you are stuck with them until they leave - you can't decide you're selling up in the middle of the 6 year tenancy.
Overall, you need to balance the advantage of the reset of the rent to market value against the restrictions you will be placing on yourself (and likely future changes to regulation).