This is a pun on the word "reign". "Reign" means to rule. The Queen has ruled England for a long time, so she has had a "long reign". "Reign" sounds like "rain". In Britain it can be very rainy, which is why they say it is a wet place. So Britain has had a "long rain" as well as a "long reign" (of their queen).
Britain is a wet place since the queen has had a long reign .