AFAIK Bromptons are like any other bike in this regard. Unless the chain is closed with a master link, you will need a chain tool to open and remove the old chain and to cut the new chain to the correct length. You will then need to close the new chain. I would recommend that you obtain a master link or a chain which comes one, for ease in future service such as cleaning and lubrication. If not you will need to close the chain by the method recommended by the manufacturer.

Here's a link to Brompton spare chains; apparently they supply them to length and also with a master link. http://brompton.com/spares/t5/chain