The most important thing that the lender is going to want to see is that you will have permanent employment when you return. If this is still in the same field as you were previously in then the lenders will be happier about lending money.
You won't neccessarily need to be on the electoral role but I do agree that this will help your case if you are. If you have a mortgage in the UK already then you will have a UK address associated with that and this also means that with the payments made that you should have a reasonble credit score too....albeit you have got no residence for the last period of time that you have been away.
It is a tricky one but not impossible, the biggest threat to you will be proving you can afford the mortgage...ie you will need permanent employment. If you can, source a mortgage broker or financial adviser local to where you will be moving to and give them a quick call and go through the scenarios with them.