transferring from US to UK

I'm currently studying at University of Virginia. I'm a first year Literature major. I'm thinking about transferring to a British school for a better English program. I've taken the SAT test, subject tests, and AP exams. My GPA for the first semester is 3.5, which is a little bit above average at my school. My favorite school is St. Andrews, but I'm not sure if I'm good enough for it.

So, could anybody tell me if I need to take some standardized tests for transferring? If I successfully get into a new college, can I transfer credits from my current school? Thank you very much! 

The only way to be sure what the process would be is to get in contact with the schools you're interested in attending. Their admissions departments and international departments will have experience in the area and should be able to answer your questions.
That said, UK and US universities are incredibly different and things like gpas and SAT scores don't have direct translations in the UK school system. The grading system is also incredibly different, which means that credits from your current school are unlikely to be accepted at any UK university. As a matter of fact, I know a few people who tried to make that transfer and ended up deciding not to because they were all told that none of their credits would transfer with them--several of them were upperclassmen and that would have meant three more years of school whereas in the US they only needed one or two.
Honestly, I considered transferring as well, but ultimately found that the better option for me was to spend a year studying abroad at a UK university and then getting my Master's degree at one later (which I am currently working on).

So, could anybody tell me if I need to take some standardized tests for transferring?

No, there are no standardised tests for university in the UK.

For UK students applying to university, they have to get specified grades in certain subjects at school in order to apply for a certain degree course.

For example, students from England who want to study English Literature at St Andrews have to get straight A's in at least 3 subjects at A level (Advanced level - approx. equivalent to AP classes or community college in the US). Students from Scotland have to get either AAAA or AAAB in their Highers subjects (which are like A levels, but slightly different systems).

According to the international students section, students from the US will need at least a 3.5 GPA, plus at least one of SATs (score of 1950+), AP classes (marks of 4 or 5) or ACT (score of 28+).

If I successfully get into a new college, can I transfer credits from my current school?

It would depend one what you have already covered in your US degree and whether the UK university will accept it as equivalent to what they teach in the UK.

Remember that degrees in Scotland are 4 years in length, and focus on 3 subjects for 2 years, then specialise in one subject for the final 2 years; while degrees in England and Wales are only 3 years in length (unless you do a placement year or year abroad) and you study only one subject for the full 3 years. This is because Scottish students finish high school at 17 after 13 years of education, while English/Welsh students finish high school at 18 after 14 years of education.

As the school systems are different between the UK and the US (and even between England/Wales and Scotland), it may be determined that you would have to start from scratch with a UK degree and your current credits won't transfer... but it will be up to the individual university.