just take your passport and visa evidence with you, everyone is really helpful. Also make sure you have copies of your CV if you haven't got a job and you can give them out when you are registering for things.
If you have kids in tow, suggest a psp or other computer thing to keep the occupied as things can take a little time!! (most banks and gov buildings have free wifi, also malls and Mc Donalds)
One thing I would definelty advice. If you are keeping UK bank accounts open make sure you can access them. We tried to pay a bill from our Halifax account in UK but couldn't as they wanted to give a verification code to a hone number (which was our old UK number).
If you have any bank accounts in UK that need 'log in devices' ie nationwide, bring that with you and you can access uk money. hope than makes sense. We have also been able to utilise our UK accounts to arrange asda delivery of flowers to grandparents to cheer them up, handy idea for Christmas.
Also bring toothaste, corsadol mouth wash and cadburys chogcolate, all vv expensive!!