It is well known that George has drawn a lot of inspiration from British history. For example;
The War of the Five Kings draws heavily on The War of the Roses and George has even stated he is heavily influenced by this.
- Houses Stark and Lannister are often compared to York and Lancaster.
The Wall is obviously inspired from Hadrian's Wall and again George has stated this.
Lots of common British medieval things have been used as influence such as tourneys, knights and what not.
Westeros looks very similar to the shape of the United Kingdom when you scan it by eye and The Wall is even similarly located to Hadrian's Wall, as are other landmarks, cities and features.
This last one is most telling and I can't remember George ever stating that he based the shape and features of Westeros on the United Kingdom.
On a similar note, and something I hadn't considered before myself, the southern parts of Westeros are almost identical to Ireland rotated 180 degrees. This appears to be more than mere coincidence and something more than just subjective opinion in the similarities.
Has George R. R. Martin ever officially stated that the shape of Westeros was based on the United Kingdom?