Keeping Credit Score Up (UK)?

After some super tricky times I have got my credit score up to a healthy 900. My house is now rented out and I am trying to sell it to buy another. I will need to get a mortgage so am interested in keeping up my credit score as high as possible. All my post is being redirected so I am still 'at' my old address but am not on the electoral roll there. Is it better to move everything over, bank, electoral roll etc to my rental property or to apply for a mortgage from my old address even though I am not on the voters roll there? Anybody know?