Most unis don't require work experience for whatever course you're applying for, but it can make you stand out and be the difference between an offer and a rejection. Some unis will also look back at what you've written about your personal statement if you miss the offer by a bit, in order to determine if they should still give you a place. I found my work experience through Access Aspiration, they're very useful as they send you emails about work experience opportunities you may be interested in. Though this is only something offered in London... I would also just ask your teachers, I got plenty of opportunities advertised to me by teachers who had heard of things here and there. Don't stress if you can't get any though, your application can still be very strong without it!