I’m a US citizen. My boyfriend is a UK citizen. I am going to visit for 6 months. But they may ask for financial proof I can support myself. But his parents can fund me.
As a US citizen you should normally be able to enter the UK visa-free for up to 6 months and be eligible to use the e-Gates on arrival at the border, in which case you may not need to prove anything. However, should you have a landing interview, you may be asked to prove that:
you’re visiting for tourism (this includes visiting family and friends)
you’re able to support yourself during your trip (or have funding from someone else to support you)
you’ve arranged accommodation for your stay
you’re able to pay for your return or onward journey (or have funding from someone else)
you’ll leave the UK at the end of your visit.
You should ask your boyfriend’s parents to send you an invitation confirming that they’ll be supporting you and are financially able to do so (the accepted way of doing this is for them to provide 6 months’ bank statements and proof of income/source of funds) What should a sponsor tell the ECO? It might also be a good idea to be prepared to explain to Border officials what you’d do should the relationship with your boyfriend break down during the visit.
Convincing a UK Border official that you’ll leave at the end of your stay typically means proving you have a compelling reason to go back to the US, a job or university studies, for example.