It is an option to keep but incurs costs, and the question is whether you want to be a landlord in this current period of time. I do it, but there are huge swathes of smaller landlords leaving the profession.
In order to keep my property, I moved out and into rented for a year, and found a tenant. I got consent to let on my residential mortgage and kept that status quo for a year, to establish a rental history.
When my residential rate was up, I moved to a buy to let mortgage, and remortgaged from my 40-50% LTV at the time, up to the maximum of 75%, and released the rest as equity. I then went house hunting with my partner, and used that equity as my deposit.
Its a lot harder to do if you are not doing the renting in the middle, as you have no proof as to what the rent you could bring in is, or if you could find a tenant, so a mortgage advisor told me that certain lenders ask for you to be stress tested as if you have both mortgages coming out of your bank for both properties.
You should check out the stamp duty costs, as you will own 2 properties, so will more than likely have a large bill to pay on your new home, which won't be incurred if you sell the house. This could easily be as large as the equity you release if you live in a low value area.
All in all, would I do it now? In my specific circumstances yes, but most other circumstances, no. I needed to move out of the area quicker than it would have been to find a house, go through the mortgage hoops and buy it, and then rent out the house. I'm now considering selling as the future is not looking great for landlords, and I could get similar growth/ return currently with a savings account.