You can apply now if you'd like to get things sorted. Write directly to supervisors explaining that you wish to work in their lab as a self funded student and when you want to start. If they like you enough they may even offer you a funded project. Some research projects can have expensive overheads (eg travel, equipment or reagents), which are not always covered by university fees. If the supervisor has time they may be able to apply for funding to cover these additional costs.
If you want more time to choose the area of research you'd like to pursue then don't worry, applying in the summer will probably give enough time for the wheels of university administration to get into motion. It will however reduce your chances of receiving any financial support towards fees.