Traveling to the UK, how much funds/documents would I need?

You were probably denied entry due to your age and lack of funds too. You only need your passport and a return air ticket. You don't need a visa. Supply one address of where you will be staying. Keep a small amount of pounds on you so if you lose your debit card you won't be broke. Just use your debit card at an ATM as you go along, but remember that your bank may charge a transaction fee each time you pull money out.
$1,200 is not much for the amount of time you are planning on staying. Think about the cost of food if you are going to eat out at all, or partying at pubs, and transportation costs if you are going to do any sightseeing. Go on line and research the costs and make a daily budget and stick to it! I would be more comfortable with 2,000 or 2500. Money goes fast while traveling. Faster than you might imagine!

Have a good trip!

Understandable, thank you. The only reason why I am not planning on bringing more is because I honestly dont plan to do all that much other than visit with my girlfriend and friends and maybe go down to the coast or a quick shop or something like that. I dont like parties and both me and my girlfriend have illnesses that make walking and running long distances difficult so we cant do a lot of things that would cost a bunch of money anyway. ^^ But thank you, I will keep that in mind.

If I say to the border control that I dont plan on doing much other than visiting people, would me having $1,200 be enough to satisfy them?

Oh yes, and I also get paid 300 a month by my parents so I'll have that added on as well.

To answer your question - you need as much as you need; but it's not only about the amount of money ($1,200 equates to £150 over six weeks, which isn't much) it's the bigger overall picture.

Do you have a job to go back to; how old are you; and indeed how would you support yourself if things didn't work out at your girlfriend's place? These are the types of questions you will need to answer satisfactorily. Border control need to know you can support yourself and you will leave the country.

As you've been denied entry previously you can expect to be closely questioned again this time.

Best check in advance with british consulate near you.
No one can reallypredict what might come up when u try to enter the uk.
You could be detained or sent back.

Do you have a return plane ticket booked? In my experience, border patrol likes to know that you also have plans to leave the country. If you show up with $1200 and no return plans, they may question you more.

Are you working in the US?

I would echo Sharon: having a return plane ticket is going to help the situation as it's a sort of proof that you do intend to return to the U.S.

You might not have considered yourself a child when you were 17 but you would be in UK immigration law and regulations. The officer may have been trying to find out who would be responsible for looking after you.

Now the concern is going to be that you will join up with your girlfriend and not return: from what you have said I imagine you are not employed at present. Also your vague plans on what you might do whilst in the UK may not help if questioned about them.

Whether entry clearance obtained in advance is advisable depends on the specific documents you received on being refused entry. I guess that you left voluntarily, Note that successful passing of this does not guarantee entry, The front line officer can still turn you down. What you are able to do though is appeal and be in the UK whilst this process runs its course.

I would expect that when you were denied entry the officer will have placed a stamp in your passport. What does it say?

Why are you getting a visa? Or do you mean a VISA credit/debit card, not the authorization to enter a foreign country? Which do you mean?

Who would you rather get your answers from, a bunch of strangers here, none of whom - as far as I know - is a UK Border Force officer, and we can only guess based on the small amount of information that you have given us; or a professional in the British Embassy who deals with this sort of thing all the time?

If I knew that there was a fair chance of being in the same circumstances again, of being turned around and sent back again, and all that money for a ticket out the window again, I know who I would be talking to.

We do our best but you need the official answers. They can answer all your questions.

I can bet that I know what red flags the border officer will see -
Possible drain on the NHS for the illness.
Insufficient means of support.
Unemployed.
Previously denied.
Risk of overstaying and remaining with girlfriend (and father and friends).
On the other side of the coin, not remaining with the girlfriend and becoming a drain on the public purse ($300 a month or £57 a week isn't very much).
Uncertainty of trip duration and specifics.

Good luck, and I hope it goes well with the Border Force.

I have visited the UK multiple times over the past few years (so many that they asked me to join their equivalent of the US Global Entry program!). I have never been questioned about money or much of anything else except why I was visiting the UK and how long I would stay there. My answers have always been short to the point and truthful.

But then I was not applying for a visa and never stayed more than a month. You don't need a visa just to visit for a month or six weeks.

If you are a US citizen and are traveling on a US passport, there must be something not quite right when the UK border agents talk to you that would make them so interested in asking you more questions. I think the fact that you were not an adult last time you attempted to enter the country was what set them off.

I'll call up the British consulate.
I am planning on getting a return ticket for a month to a month and a half later. I have my own little business (Nothing gigantic, just making a bunch of stuff. Sending it out, all that fun stuff.)
Yes it probably was mostly because I was 17, since a lot of the time what they did say was "We dont know if we can leave you in the responsible care of your girlfriend" Probably just because they were worried about me not being a proper adult yet.
I believe I still have the documents from when I was refused entry, but I am not sure. It was a while back. Although I'll check my passport and get back to you all on what it says.
I am getting a visa because when I was in border control (I was there for like 27ish hours) They said that the next time I come, I really should have a visa with me and at least the funds to either get myself to another place to stay (In case something happens with my girlfriend) Or get a flight home. Which with my 1,200 is more than enough.
Would I end up getting more into their favor, so to speak if I made a bunch of plans while I was there? Had a travel itinerary or something like that which said where I was going to go how much I was going to spend and that I was going to go places?
Thank you all.

I no longer have the documents from when I was sent back, my passport does not say anything other than the date and the country with an x I guess through it.
And calling the British embassy doesn't help, they just refer you to a website that I've already read and doesn't tell much.

Hi-

So I visited the UK for four months while I was between jobs. I had some money saved up and I also got a letter from my UK friends saying that I was staying with them, where that was, and if there was any emergency in which I needed more money, they would help me. Now that isn't required, but it could be helpful.
I was advised to get a visa for those 4 months- BUT you do NOT need it. I didn't even need it and it almost became a burden at security. One of the border control agents told me after the fact that they actually get a lot of people who have been denied entry trying to get in with a visa, and that is why they questioned me so much about having the visa for no reason.

Now this time, don't babble. Only answer questions they ask with just the facts. Remain quiet until they ask you another question :) And have a copy of your flight itinerary for both flights. It's the return flight they care the most about. If you are not currently employed, have your response ready as to why. If you are employed, they will probably ask you how you are able to take that much time off, so have that ready.