Usually broadband and mobile contracts are separate so you should be able to. If you go into your BT account you can check the dates. Also BT usually send emails when contracts are due to end with new deals etc.
With my contract ending soon,it includes a mobile sim,am I able to cancel broadband and just keep the sim ?
You can but any broadband benefits will stop, so that means the mobile will increase by £5 per month, any Halo benefits like double data, fastest 4G speeds and other perks would also be removed.