As RedOnion says, you're under no obligation to keep the existing property, so long as you keep paying the mortgage, they'll leave you alone.
I'm still looking, but the approval I now have in place assumes I will rent out my existing house, and I've been approved for more than I was based on the option of selling it with a percentage of the expected rental counting towards income calculations.
If and whenever I do finally buy somewhere, I'll keep my current place to give me time to move without the need to arrange storage. I'll decide then whether to rent it or sell, the decision likely being driven by whether I need to free up money for any renovation work required on the new place. Note it will be very important that I keep sufficient cash reserves to allow me cover the new mortgage and several months of the remaining mortgage on my existing house. If you're going down the same path, be sure you do likewise, and don't lose track of spend kitting out the new place.