The sad fact seems to be that if you show any purchased films outside the home then you are technically breaking copyright and could technically be fined £5000. That is the case whether you may want to show a full film to a young people's group, or even a 30-second clip of something as a sermon illustration.
That gives you a simple choice; either break the law, or get yourself an appropriate license. An annual licence will cost you between 2 and 4 figures depending on the size of your largest congregation and the nature of how you want to use them; There are several licenses that you need to consider purchasing, all outlined at https://uk.ccli.com/copyright-licences/
You can see what prices you need to pay by looking at; Copyright Licensing [UK]
At this stage you will have to check whether you already hold a CCL licence for reproducing song words. If so, you will be charged pro rata until your CCL license expires, and then be asked to renew them simultaneously.
You would also be well advised to check out the ‘Screenvue website'
If you'd like to look at any of this in any more detail, we suggest you look up; Church Copyright Licence [UK] Copyright law is a complex area that we cannot hope to cover in one simple document, but hopefully this will at least tell you the essentials of what you need to know.