Is this a bad plan?

It's not a "BAD" plan--it's an uninformed one. You need to do a lot more research before picking up and moving to England to do ANYthing. Jobs there are less available than they are here. Universities are much more difficult to attend. You  haven't even mentioned what you will do with your citizenship once you marry this man. I would not take any loans out now--not before finding out a lot more answers that you don't have yet. What will happen to you if you DO take that loan and then find yourself unable to find a job, get into school or marry your boyfriend? You're thinking with your heart--not your head. I don't care what you refer to yourselves as--legally that means nothing at all. Find out more info first. 

You won't be accepted into any of the universities in England. You never should have earned a high school diploma because you don't know the difference in usage between "there" and "their."

the mind goggles , you have 17,000 and your education will cost you 80,000 but plan to get married when you finish school , good idea except how do you plan to pay the loan , and if he likes america while your in england , shakes head and thinks what an idiotic idea ..

Don't do it. You will severely ruin your life. Yes, it's ridiculous. 

At 25, have you not already studied in the US? You won't get a student visa to study at the same level as you've achieved before. 

You're also planning a lot on something that might happen in 4 years... what if it doesn't? Will the qualifications you get in the UK work in the US if plans change and you return home?

The only loan you'd get would be a private loan; and £80k is a lot to borrow. You may find you can't. 

in England you are allowed to work 20 hours per week as a foreign student. And also you can work   

any hours between the semesters.

Marrying him will not make the cost of studying cheaper.  To be a home student (meaning entitled to the lower fees and a UK student loan) you need to have been UK resident for the three years prior to the start of your studies AND not need a visa meaning ILR or citizenship.  You qualify on neither grounds. 

You will not get a student visa unless you can show you can pay for your studies.  You could get a fiancee visa and enter on that but it would require you to marry within 90 days or go home. 

One thing you might consider is applying to a UK university that is FAFSA eligible.  The list is on the FAFSA website. 

You will not get into any UK university to do an undergraduate degree with just a High School Graduation certificate.  FAFSA eligible ones will all require APs or an Associates degree.  UK undergraduate degrees are 3 years, not 4; narrower and deeper than US degrees.  You select your subject before you start and study only that one subject, generally with prerequisites.  I found that my husband's 4th year studies in the US were the academic equivalent of my 1st year studies at university in the UK.

Finally, please don't refer to one another as husband and wife.  That downgrades marriage and will upset those around you.

I don't know of any place that will give you a loan to study abroad. Student loans are for studying in the US. What you can do is enroll in a US university and use their study abroad program to spend some time in England. It's much cheaper, and your financial aid will pay the tuition, though not the transportation and living expenses. Eventually, one of you is going to have to bring the other in on a fiancee visa. In the US, that gives you 90 days to get married.

I agree with the other responder that you'd do better to go ahead and get married. Then you can be a resident and pay much less for your studies. 

Do not  refer to each other as husband and wife.

It will only cause confusion.

You are not going to be able to study in the UK.  Between the pandemic and because you simply do not have enough money. You will not get a student visa. You will  not be able to take out a student loan to study in a foreign in a country. And without a lot of collateral (like owning a house)  no one is going to give you a personal loan.  Especially for studying aboard. 

Your best bet is to get your degree in the USA. Get your degree then get married.   

My friends got married five years ago. She is here in the USA and he was in England.  They got married in the USA.  And live here now. This is a very long process, it involves a lot of red tape to go through and it is expensive!!!  You need to be interviewed, he needs to be interviewed, you need to provide documentation, bank statements, proof of how long you have known each other, proof of any visits.   After they were married he still could not work for another six months. You going to England isn't going to make it any easier or cheaper to do. 

My niece is married to a guy from England.  They are in limbo right now and have spent many weeks apart. To stay on England they must earn a specific amount of money, which they don’t have. They up and got married in the USA on the spur of the moment. Then tried to go through the process of visas and moving. Do not do it this way. 

And unless you are married first, chances of you getting a job and then moving are slim to nil.

You better take a good long look at the reality of moving to England, getting married there and getting a job there as an US citizen.   Do your own research on it. Many couples end up hiring a lawyer/solicitor to help with the process. 

"Boyfriend of 6 years" is not good.  If you don't have a ring on it after one year, that's a huge red flag.