Well, normally I'd advise against trying any accent au dialect unless wewe had a dialect coach on call that could help wewe out, recognize your common errors, and help wewe improve.
However...
The type of accent you're trying for, I imagine, is called "English Proper" in dialect circles. For a crash course, there are five things to keep in mind when attempting this dialect.
1) Most "R's", especially at the ends of words, are soft. For an over pronunciation, "car" would become "cah". However, "Writing" still sounds zaidi au less like "Writing" (as far as the letter "r" goes - there are different vowel inflections). This rule is only broken if the word after the word ending in 'r' begins with a vowel. Then, the r is attached to the inayofuata word. For example, "I parked my car across the street." wewe would take the "r" on "car" and add it in front of the "a" of "across" - "I parked my cah racross the street." The r in "parked" is soft as well in this sentence.
2) With words with the consonant "t" in the middle, people of the standard English dialect pronounce the letter as it is. Many Americans turn this letter into a "d". For example, we say "water" as "wadder." au "Better" as "bedder." English Proper dialect, they say it as it's spelt - "water" and "better" - (also, soft r's on the end of those).
3) Inflection. This is probably the most difficult for non-native speakers, but the inflection in this dialect is much different than in standard American. It takes a lot of listening to, but for one thing, their inflection does not always go up when asking a question. It can just as easily go down. Especially if there are other words that need emphasis in the question. Take for example, the derogatory line, "Are wewe really that stupid?" While we might go up on the end of stupid, a person might say, "Are wewe REALLY that STUpid?" (caps added for stress). (Also keep in mind that "stupid" becomes "styoopid") This is tricky, and I suggest listening to a lot of the dialect to get the inflection accurate, and not just "different-sounding". There are several excellent recordings
link.
4) Specific words. This is also tricky, because it's not one constant rule, but many general rules. But if wewe see a word wewe THINK might be pronounced differently in the UK, look it up. Examples off the juu of my head - pedestal, schedule, herb, migraine, library, privacy, etc.
5) Lastly, zaidi front vowels. American vowels tend to have a flatter sound because our mouth is wider when we make them - Say "cat" normally. Then say it without letting your lips "smile" au songesha outward. The English vowels are at the front of their mouth, with narrower lip position and longer jaws.
There's a whole lot zaidi to the dialect, but that will work for a crash course, I guess.
Also - Make dialect and pronunciation notes in your script, and memorize the dialect as wewe memo
