You don't need GCSE certificates to apply through UCAS. As the above poster says, you can get replacement certificates for a fee from the exam boards. However, universities themselves may request copies of your GCSE certificates, either before or after giving you an offer, or later or in the application process. The vast majority of my friends did not have their certificates asked for, but most (if not all) of my universities wanted copies as I was applying for medicine. Employers may ask for certificates, so I suggest that you order replacement ones.
There is no need to panic as although replacement certificates may take a couple of months to come, it is likely going to be at least a couple of months after submitting your UCAS application, so you have plenty of time to order new ones. Plenty of people lose them, so don't worry :-)
Although your school can probably give you your statement of results, you need the actual certificates as proof.