In terms of the postal vote, there are forms to fill in to register, or contact your local council for assistance and advice.
I know there are clever websites that show you how to vote where you live if you want to unseat an MP (fat chance where I am; it basically says don't bother!) but you can just look at Wikipedia , tbh , for majorities.
So...for example, where DS lives now has a 21.000 Conservative majority. Lincoln, where he is headed, has a Labour MP with a majority of just over 1000!
Annoyingly where I live, if I lived less than a mile up the road , I would be in another bellweather with a Labour majority of 2000!