When you have found a property and you are ready to move, you can transfer your existing mortgage to your new home which is known as porting or, you can take out a completely new mortgage with your current lender or a new lender.
It is best to know how much you will be approved for before finding your dream property. When you apply for a mortgage the lender will offer you a mortgage in principle (or a decision in principle), this is a commitment for them to lend you the money you have requested (assuming the information you have provided is accurate). Most home sellers will not accept an offer on the property unless you have an agreement in principle. The agreement in principle will typically last for 90 days before the full mortgage application will need to start.