Yeah, Tyrion's accent it truly appalling. The reason everyone has English accents, I suspect, is because the whole thing is based upon English history. Although not following history exactly, the Lannisters roughly correspond to house Lancaster and the Starks to House York. There are some tips of the hat to the Saxons, Normans and Celts, too.
Originally Posted by HarryT
One question I have:
This is an American fantasy series, set in a completely fictional world. Why does everyone put on (sometimes rather bad) British accents? Has LOTR made people expect that all fantasy characters speak with British accents?