Yes and no. All universities are listed on UCAS and all formally accept applications through the UCAS platform. That being said, you can apply directly if you need to. You generally do this when changing universities mid-term, rather than part of the initial application, but you can apply directly to some. Some universities do not allow it, however. A select few universities may potentially reject any offers that do not come through UCAS, so best to stick with UCAS, if you can.