The OSS pledge to embrace the rejuvenating effects of cold water is a valuable mindset change for any open water swimmer. At first it may seem inconvenient that British waters are not warmer, but you may soon find you're addicted to the fresh feeling of natural water.
The good news is that it's relatively easy to acclimatise yourself to water temperatures in the UK, by nothing more complicated than swimming regularly in them.
The human body acclimatises to cold water so well that hundreds of swimmers cross the channel every year (at around 16 degrees, for 10 hours or more, an untrained swimmer would get hypothermia), and a few swimmers (most notably Lynne Cox and Lewis Pugh) have even trained themselves to swim in the polar regions. This has led to the Ice Mile event, where swimmers take on a mile in water at 5℃ or lower. Such feats require expert knowledge and training and are potentially extremely dangerous, but it is possible.
Getting yourself into a situation where you enjoy summer swimming, particularly if you're prepared to start out with a wetsuit, should not take long.