To apply for your hgv class 1 licence (C+E)(ARTIC) you should have already passed your class 2 (C)(RIGID) test first and received your licence back. Dvla will add class 1 provisional licence to your license so you can start training. There is a variety of different professional hgv schools out there, they can assess you before booking your lessons on how many hours, you will need normally the hours are between 12-20 hours depending on experience, learning capabilities and the cost should be between £950-£1500. You should take one to one sessions instead of learning with another student. This way your instructor and you can focus on your strong and weak points.
There is a lot of good hgv training schools all around the uk. You should visit the school you may choose before handing any money over. You can also see the lorries and what the school is like and that they are not brokers and a actual school who will train you. You could ask to meet the instructors that will train you.
Good Luck With Your Test!