If you wish to race in England, you will need a letter from your countries national association, which gives you permission to request a British race licence. Then you need a medical, RYA affiliated club and a British licence to race in a national race.
There are however BASIC races, which do not require such formalities, and for which you can buy a licence on the day.
These are often more fun than the national races!
The most interesting race in my opinion is the COWES BASIC. Another good one is the Cancer Charities or the Brass Monkey.