You can apply directly for jobs. You do not need any particular qualifications, although some employers may want to see a GCSE at grade 9 to 4 (A* to C) in English.
You'll need to be able to deal sensitively with bereaved people. It can also be useful if you have experience of working with the public and basic IT skills.
Once you're working in a crematorium, your employer will arrange for you to follow the Crematorium Technicians Training Scheme.