There have always been mosquitoes in this country. (In fact, there are 32 different species native to Britain). Most of them don't bite humans and, fortunately, those that do bite don't carry diseases like malaria.
While global warming will inevitably bring changes to the populations to the populations of many creatures, you'll always find more mosquitoes when conditions are damp and humid. The recent damp and 'sticky' weather is nothing unusual for a British summer.
Chris