i am a bit unsure about the suits when diving when i do my first dive in really cold waters in england can i get away with a semi dry i know this is silly to do i need to wear an undersuit as well

I know the water in England is very cold. Some dive with semi-dry and you can get away with it. But many dive with dry suits. You will stay warmer and have more enjoyment in the dry suit.
You can wear a dive skin and/or bathing suit under the semi-dry suit.