[RobertP]

Speaking as someone who employs people in a small factory it is rare to receive a written resignation. Usually they just tell the supervisor who then informs me and my business partner.
If the person leaving is a valued employee you talk to them and try to find out why they are leaving. If you can sort out the reason you do.
If the employer has any sense they will try to change what has made the job so bad for you.

Don't think we have ever had anyone leave on bad terms - unless you count the one I sacked. We have also re employed at least 2 people that left then came back a few weeks later.

A letter is the correct way to do it but not essential. You should expect to work a notice period and you will get any holiday pay owing. A 'career improvement/experience' , 'going because you need the money' , 'less travelling'.... are all the sort of reasons that would not burn any bridges.