Being that you plan on staying for such a long time in combination with the fact you've admitted you plan to possibly eventually marry, it would be wise, in my opinion, to apply for a visit visa in advance like you mentioned.
You want to keep some things in mind, which I'm sure others will point out as well or even hopefully add to...
First of all, since you're leaving your job before entering the UK, you need to show clearly that you plan to return to the US. In most cases, this would be a letter from your employer stating you have a job to return to. Since this will not be the case, you need to find another way, such as a mortgage agreement/property you own and will need to return to or something else tying you to a definite return to the US.
Also, have you been to the UK before, or will this be your first visit? Do make sure you and your boyfriend are both on the same "page" in terms of your long-term plans. If you're questioned on arrival in the UK about your intentions, don't volunteer any information...but if you're asked point-blank if you intent to get married, make sure you make it clear that you know the rules and know that if that does become the plan in the future, you must first return to the US to secure the proper entry clearance.
I can't think of anything more off the top of my head, but like I said, there are many people here who have been in the same boat or are even more familiar with how you should proceed.
Good luck!