There are 20 odd species of Mosquito in the UK. Midges, Horseflies and Ticks are often confused for Mosquito also.
Any blood- sucking insect is capable of causing a mild allergic reaction. No UK mosquitos are thought to transmit diseases presently.
I'm always a target for mossies, but have only ever experienced the the usual itching & scratching from their bites.
However, I came home from Cyprus with a few mossy bites & the one on my arm became infected. A large lump appeared from it & if I brushed it on anything, even coat sleeves, it would pour with blood. I had to have it removed under a local anesthetic & ended up with three stitches!