Welcome to the forum. A few thoughts from someone who knows very little about graphic design!
Local is always good because you can go see them if they are interested. You aren't too far from Nene Valley - why not try them first? And maybe Great Central as well. If they are like our local operation (Watercress) they will have a standard volunteering form and an email address to send it to. Start by giving them a short resume and ideas of what you could do. Remember that they get any number of applications to do something around a steam train (Watercress's roster for volunteer loco cleaners is full until autumn) so make it short and sweet and stand out from the crowd. If local ones don't work it might also be worth you looking at websites and find one that looks a bit amateur!