Not quite sure what kind of answer you want, Barquentine. You might just as well ask why Russia only has 3 recorded cases.
It is unlikely to be a single reason, but more likely to be a combination of reasons, such as those I have listed below.
1. Wider initial exposure to the virus -
Would promote a greater and faster spread in the population - So, it might simply be that, initially,many more Brits were in Mexico / Canada than other european nationalities, and thus exposed to the virus.
2. Testing procedure -
The gold standard test is PCR - some countries have made extensive use of a rapid version of this test, which is less accurate, so more positives will be recorded.
3.Reporting procedure -
Now that diagnoses are being made on the basis of a list of symptoms, it could simply be that more people in the UK are inclined to report their symptoms, be diagnosed as having H1N1, ( possibly incorrectly), and put on the list.
4. Cultural attitudes to preventing virus infection -
At least one poll has suggested that 75% of Brits have not altered their hygiene routines at all, even though official recommended advice would suggest more frequent handwashing, for example.