Do I need to register with HMRC every year?

No. Once you're registered, your UTR stays the same and you just file a new return each year. You don't need to do anything differently from year to year in terms of setup. The only exception is if your circumstances change and you no longer need to file — in that case you should contact HMRC to ask them to remove you from Self Assessment, otherwise they'll continue to expect a return from you each year.