Each exchange program has its own application deadline. The UK application deadline is set 3-4 weeks before the exchange host institution deadline.
Some exchange host institutions will process applications and officially admit students on a rolling basis while others wait until an official deadline before starting this process. In some cases, you may have to wait until relatively late in the semester (or sometimes after the end of the UK semester) to receive official notification of acceptance.
Don't worry, this will still give you enough time to complete all necessary pre-departure requirements. In both cases, we recommend you get an early start on your UK application. There is a lot to get done and some exchange programs fill quickly.