If you are going to move to Newcastle, then I would personally have the wedding in South Wales on the basis that you will see all the family and friends in Newcastle all the time, but if anyone from South Wales can't afford the journey down to Newcastle, you missed that opportunity to spend some quality time with them too. If you do something in the middle you are costing everyone money, as opposed to just one side. Or figure out which side is more likely to take the time to travel, only you know your guests.
Figure out your headcount, look to get married out of season and maybe on a week day, all these will keep the costs down. Then maybe find a hotel that does a special deal, that can offer affordable accommodation on site, then have a twilight wedding so you only have to feed everybody once. These are just a few things to do to save a few ££'s. Good luck x