In short yes. If you’re planning on becoming a PT resident, you’ll need to import the car into Portugal and switch the van over to PT plates.
Most people import cars and this can be tricky. I would actually get in touch with an agent for a quote in advance to find out how much you should expect to pay in ISV tax. There is an exemption period for new residents if you do it within a specific timeframe. Not sure if it applies to vans.
I don’t think you can not import it if you’re a resident so that’s not an option. The only other option would be selling it for scrap or finding someone going back to the UK who wants one.